Colouring technical illustrations

Silly me

He has ‘New Gamboge’ on there as well as ‘Indian Yellow’

MichaelG.

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Most of the makers do two colour charts, one printed which is quite well colour matched and then the expensive chart which is made up of individual pieces of water colour paper with a graduated wash of each colour and these are obviously the best guide to what may come out the tube of off the block. Once you add in screen and scanner variations you get further from the true colour.

Thanks for mentioning it, but

I don’t anticipate buying the ‘Analogue’ version of that chart, Jason

Eventually, they say: Curiosity killed the Cat

and I’m happy to keep this quest for knowledge at level I can comprehend and afford.

MichaelG.

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FootNote:

For anyone who wants to join me in that quest: Here is a crop of the ‘New Gamboge’ from that chart, with the worst of the jpeg artefacts at the edges omitted.

N.B. This will be a lamentably poor approximation of the real thing, but we can now set-to with the digital toys.

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Edited By Michael Gilligan on 03/11/2022 09:39:57

It's already got too complicated for me! The RGB system is an excellent way of defining colours numerically, but it runs smack into the buffers when trying to predict the result of mixing two known colours. Mixing Blue and Yellow produces Green in our heads, but adding RGB values doesn't. It seems there isn't a simple algorithm for mixing RGB color codes and getting the expected result.

So. colouring with a computer, I don't know of an easy way of calculating the colour of Brass - 'Gamboge with a little Red'. No bother with a kiddy paintbox but I can't program it. Doesn't matter if the colouring is done with paint on paper, but it's much easier for a cack-handed brute like me to colour digital images with software.

Dave

Here’s a starter:

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Dave,

I have an assortment of Apps which look very promising

Having worked at KODAK in the 1970s … whilst I am certainly no expert, the general subject of colour reproduction fascinates me and I suppose this thread was prompted by that.

Happy to continue here, or off-forum if everyone else is bored already.

MichaelG.

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Edit: __ 1931 was probably the year that practical things started to get technical:

Edited By Michael Gilligan on 03/11/2022 10:14:03

I just gave it a try in Alibre, although I can enter the RGB values which I took from Dave's earlier link that gives a very solid colour much like it would come out of the tube.

However once I move the slider to get a lighter "wash" the RGB values change so is it still Gamboge?

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The lighter “wash” should simply be the result of all three values changing in the same proportion.

So, Yes [within the bounds of what the bit-depth can handle] you still have the same colour

Does Alibre mention what ‘Colour Space’ it uses ?

MichaelG.

Edited By Michael Gilligan on 03/11/2022 10:44:20

This is the output from an App which analyses the content of an image, and shows all the colours used within it:

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Plugging any of those numbers into Alibre, or whatever, should produce the same colour as the adjacent patch.

Remembering of course, that what you see on your screen will inevitably differ from what I see on mine.

MichaelG.

This is a long way from the 2 ladies who used to sit close to me in Timsons drawing office in the mid sixties. Timsons built bespoke printing machines, and Jess and Jean were employed to prepare the proposal drawings. Jean would use her box of Windsor and Newton water colours and a fine brush to colour in the drawings, and Jean would annotate the drawings in black ink in beautiful copperplate. We had a very demanding boss, and sometimes the drawings were re-done several times to make sure the information was understandable. Here colours did not matter so much, as the drawings were not depicting the materials used, but the functions of the various parts of the machines.

Chris Gunn

I'm not sure of the "colour space" that it uses.

I've done a little video which will compensate for screen differences as it will all be on the same screen. On the left is a part coloured by entering Dave's RGB values which can be seen on the right side of the screen.

I then get Michael's recent post up on the right and move the slider in Alibre up to "lighten" the colour but the RGB value change is certainly not proportional.

Finally enter half the RGB values and end up with poo brown rather than a washed out gamboge. I seem to remember mixing Plasticine as a kid had similar results!  If I double all the values I get canary yellow. The very lightest tint from the slider is something like 255, 254, 253 and the darkest 9, 5, 0 before going to white or black respectively.

Edited By JasonB on 03/11/2022 11:46:04

Sorry, Jason … I see the problem

Your slider is [quite properly] adjusting the saturation of the colour, which is like adding water to your paint.

… when you are mixing light, the result can be expressed in terms of Hue, Saturation, and Brightness

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I would struggle to describe it in a forum post, but thankfully Wikipedia does a pretty good job, here:

**LINK**

https://en.wikipedia.org/wiki/HSL_and_HSV

MichaelG.

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Incidentally, the ‘Colour Space’ determines the available gamut [palette] for display

… in my 2D, posted earlier, it is sRGB and that’s what determines the particular shape of the triangular area on the graph.

This is a screen-shot from the [quite astonishing] Concepts App, which I am currently trying to learn to use.

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The colour half-filling the ‘sight’ was selected using the eye-dropper tool, and the picker menu is currently displaying HSL … alternative displays are RGB and COPIC

This is just the way of selecting the colour for drawing/painting on an ‘infinite canvas’ … there is much, much, more to the App.

MichaelG.

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Ref. __ https://concepts.app/en/ios/manual/yourworkspace#thestatusbar

Edited By Michael Gilligan on 03/11/2022 13:31:04

Although computer colour codes are often given as a triplet of hexadecimal numbers representing Red, Green and Blue, software usually silently does RGBA, in which a fourth hex number is added to control transparency, or 'alpha'.

As Jason said, proportionally halving an RGB triplet doesn't produce a lighter shade of the same colour, which is what I expected. Saffron as an example:

Halving the RGB values of Saffron to 7a6218 produces a mucky khaki like colour. As it doesn't seem to have an official name, I suggest "Duffer's Poo" in my honour, and, by popular demand, all model locomotives will be finished in it.

F4C430 is still Saffron, but F4C4303F is the shade of Saffron produced by being 50% transparent. In hexadecimal half FF is 3F. Hex reveals the RGB values, the same number in decimal (16041008) has to be decoded.

Michael mentions several other systems - they all make my head hurt!

Dave

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Thanks for that succinct explanation, Dave

3F duly noted, for my continuing attempts to learn something.

MichaelG.

Dave,

Half of FF is 7F (well 7F.8 to be exact) - so I assume that 3F represents 25%.

Which brings the question 25% of what? Is it 25% "pigment", implying the remaining 75% is transparent?

Would this then be described as 25% or 75% transparent - a problem neatly avoided by using 50% as an example!

Keith

Well spotted Keith, the mistake is doubly inexcusable because I used a calculator! I guess I double-clicked.

Anyway I'd describe #3f as being 25% opaque and 75% transparent because screen colours are like the projection on a cinema screen, illuminations.

Counter-intuitive I think. Illuminations are opposite to normal vision where our perception of colour, such as admiring a flower, results from the brain processing light frequencies that have been absorbed. Mixing pigment colours causes the result to get darker, whilst mixing coloured light causes the result to get brighter. The two ways colours work need a mental back-flip like reversing a car with a mirror, or undoing an upside down bolt.

Being illuminated means a computer screen isn't really opaque or transparent, rather it's an illusion where brightness is increased to make lighter shades and reduced to make darker.

Vision is very strange. Black absorbs everything, so it's not really a colour at all. We seem to detect the absence of light. Even more confusing, we can't see it! Light travelling through space is invisible until it hits something. What we perceive as shape and colour are always the result of reflections. It seems shapes and colour are the product of our brains making sense of differences due to light bouncing, or not bouncing. Another oddity is that the image in our eyes is upside down, and this is corrected by the brain so we fink the world makes sense. However, when someone wears a pair of inverting specs, the world is upside down until the brain relearns and we adapt to the glasses. When his inverting glasses are removed, the victim sees the world upside down until the brain adapts back again, which takes about an hour to adapt. It means that what we see isn't really real, it's just a useful approximation!

Dave

We are talking about drawing-office practices from 100 and more years ago. I wonder how "standard" the colours were, since things like RAL values and digital versions were yet to come.

I suspect the procedure was acquire the colour from the stores, mix some and maybe check with the artist next to you if it looked right.

Edited By Nicholas Wheeler 1 on 03/11/2022 18:18:27

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That is exactly why I asked the opening question, and why it is [for me at least] necessary to use the tools at our disposal, to help understand what we now see, and how it might compare with what they intended at the time.

When I returned from this afternoon’s shopping trip, there was a very interesting eMail from my friendly Librarian at ‘John Rylands’ … she has located a book in their vast collection, by William Savage, called ‘Practical hints on decorative printing’ dated 1822.

To quote her first-hand observation:

“ Your Gamboge appears to be a faded lime green on their plates ! ”

Now … at 1822, this is probably natural Gamboge … since which time, there have been several attempts to produce a convincing and stable synthetic alternative.

”Our” illustrators were probably [consciously or unconsciously] in the first wave of those using Analine/Azo dyes and pigments.

All I can do, Nigel, is repeat that I find this interesting and worthy of some research.

MichaelG.