Tuesday, December 13, 2011

Warhammer and painting - why use a RYB colour model?

As I've mentioned I did some Christmas shopping for Bratus Minor and I did end up buying him a "How to Paint" book. Prior to wrapping I had a flick through it and found they were using the Red Yellow Blue (RYB) colour model on how to mix paints. This is a popular model that painters use; but why?

As part of an entry on computers images I dealt with colour models, but a quick recap should suffice for this:

If you're reading this on a computer you will already be familiar with one colour model even if you don't realise it. Televisions; computer screens all use the Red Green Blue (RGB) model. This model is known as additive (I think of it as emittive because it emits light). Using this model if none of the three named primary colours emit light we perceive it as black; if all three emit light at the maximum degree we perceive it as white.

Another model will also be familiar to those who use computers particularly printers - the Cyan, Magenta, Yellow (CMY) model. This is known as subtractive (again I think of this as reflective). This is the opposite of the RGB additive model in that with none of the primary colours reflecting light we perceive the colour of the surface (normally white paper); if all three are mixed we perceive it as black.

Dependent on how we are using colour either of these models suffice to present a large range of colours by the careful mixing of any of their three primary colours. Given that why bother with a RYB model at all?

It's all to do with how colours mix. In the RGB model if we emit equal quantities of Red and Green light we perceive Yellow. If we mix equal quantities of Cyan and Magenta we perceive a Blue reflection. So far so easy. However what happens if I add a primary colour to a mix of the other two?

In the case of RGB, adding Yellow (Red+Green) to Blue should produce White and it does (or at least a grey). In the case of CMY adding Blue (Cyan+Magenta) to Blue should produce black and it does, but only when dealing with pure colours. But pigments aren't pure.

Add all the colours of the CMY model and you don't get black you get a murky brown which is why on printers they add the K or Black. Take into account the pigments found in paints etc. and what would be predicted from the CMY model doesn't always work. So add a Blue pigment to a Yellow pigment and rather than black or even the murky brown you perceive Green. It turns out that the model that works closest with actual paints is the RYB model.


Anonymous said...

Definitely a good look at it. Personally though, I'd say it depends on the paints you use. There are plenty of paint sets you could assemble for a CMY wheel to make whatever hue you wanted. I use it as a more reliable way to (try to) find hues without surprises from mixing, like getting the wrong purples or greens.

FlipC said...

To a certain extent I think it's a matter of tradition. The RYB colour model has been in use for a long time thus pigments are mixed to match that scheme, which means having to use RYB :-)

As an aside in the CMY printer system black is rarely black but simply a very dark shade of a colour. The manufacturers know this and match their specialist paper to suit. So use Printer A with Printer B paper and blacks may appear greenish because A uses a dark blue for its black whereas B uses dark green.