Building a Digital Colour Wheel

Most of us are familiar with the conventional colour wheel. It is the colours of the rainbow wrapped around and joined at the purple/violet segments. It normally shows 12 colours, the primaries (Red, Yellow & Blue) then the secondary colours easily mixed from those (Orange, Green & Purple). Finally the six tertiary colours mixed from the adjacent primary and secondary colours. Whilst many people instinctively know a harmonious colour scheme (eg red & green, Blue & Orange, Yellow and Purple) They might find it difficult to describe why. The tradition colour wheel can come to the rescue here, colours on opposite sides of the circle are called complementary, beside each other are call analogous, In addition if one of the complimentary colours is left out but the analogous colours either side of the missing compliment are present then this is described as a split complementary. All these combinations are know to be harmonious (and desirable for an artist, photographer, home decorator or fashion designer). The colour wheel is very useful and you can read a bit more about Basic Colour Theory at Color Matters.

Some complications arise when you start to look at how cameras and TVs work. They use a three colour palette RGB (Red, Green & Blue). This doesn’t fit onto the traditional colour wheel very well (first image below). These three colours are also know as the additive palette. These three colours do work well in terms of how our eyes (specifically the three types of  “cones” in our retina) recognize colour. A lot of people not familiar with this system are very surprised at the lack of yellow, but if you add red and green light sources you will see yellow. Things get even more strange when you consider the approach developed by traditional colour printers (including your ink jet printer) which use the  CMY [K] (Cyan Magenta & Yellow) subtractive palette. The K in square brackets stands for blacK, as printers find they need a true black, because a mixture of magenta, cyan and yellow inks or dyes tends to be a muddy dark grey and leaves images flat. Black is not really a colour, but I wish to avoid the argument on that for now. It is even harder to fit these colours on a traditional colour wheel (there is no matching segments for Magenta or Cyan for a start). You can read a bit more on the RGB & CMYK colour systems at Colour Matters.

This is the point where I decided to combine these two palettes, and form 6 new primary colours. I’ve since discovered I’m not the first to have attempted this (eg see Warren Mars website) and this configuration is often known as the modern or digital colour palette, sometimes even the RGBCYM[K] palette.

You can then fill in the intermediate segments with new secondary colours to get a simple 12 colour wheel again. Amazingly if you employ the idea that opposite colours can be complimentary or adjacent colours analogous and you find they are also harmonious. What is going on here is the traditional colour wheel wrong? Perhaps for both their photography and printing aspects digital photographers might be wise to adopt or at least consider this new colour wheel. I will be discussing many of the issues for digital photographers in coming blog posts.

The very last image shows the colours Adobe Photoshop and Lightroom give you control sliders over. I’m still no closer to being able to explain this colour selection.

Towards a new Colour Wheel, but Why?

For a while now I have been bending the ears of my colleagues and friends incessantly about a “digital” colour wheel with 6 primary colours rather than 3. Looking back in the blog post I  hadn’t posted anything in here (yet!) So now is the time to fix that.

	Adobe Primary & Secondary Tertiary Red Red Red Orange Orange Orange Yellow Orange Yellow Yellow Yellow Green Green Green Aqua Blue Green Blue Blue Blue Violet Purple Violet Magenta          ? Red Violet

I started to personally  question the colour wheel adopted by adobe ( within lightroom and photoshop) for their omission of some colours in their digital post processing. It only gave me 8 colours, halfway between the convention 6 primary & secondary or the 12 primary secondary & tertiary colors I expected)?

Why have they done that?

So I looked at the ways are colours are handled in the camera, computer screens and then onto printers. Well they are not even in the same colour spaces, or built using the same colour models. Cameras and Computers screens (and any direct transmission of light) follow the RGB (Red Green Blue) colour model.  This an called an additive model of how colour works, because different wave lengths of light get added together. This somewhat matches the rods in out eyes which give use colour perception. We can only discern colour because different cones in or retina can detect different wavelength. If you mix projected light of these three colours you get pure white. Ok good but should that be yellow rather than green (green is a secondary colour isn’t it?). Damn if I mix green and red light I can get yellow?

Things get even more bizarre when you start looking at printing colour.  Traditional three colour printing and many inkjet printers use a different set for their primary colours (CYM) Cyan Yellow and Magenta! This is called a Subtractive colour model because certain wavelengths are removed from the reflected image. What is going on here? When in comes to reflected light and pigments mixing colour is again different, for example to get green you mix your light blue (cyan) and yellow!! If you mix all the three colours you get a dark muddy colour, close to black but not a true black. So many primers (and ink jet manufacturers) also use a per black to create the CYMK (K for black) scheme.

Close to a year ago I was doodling in my sketch book, and realized by combining these two colour schemes I had a new colour model with 6 primary colours. Not surprising, the idea of a RGBCYM[K] colour scheme has occurred to others and is sometimes called the modern or digital colour wheel.

I know I haven’t answered the big question why Adobe and many other photoeditors only give to eight color sliders. I can’t find out why (at the moment). This is a big topic so I will build and develop over several blog post to come.

Other Blog posts in here around the mysteries of Digital Colour Theory

• Building a Digital Colour Wheel
• Just How Many Colour Models/Spaces are there?
• Getting more Colours with Bit Depth
• The Gamut of Colour Spaces
• Chasing the Missing Colours
• Colour Names and the Martian Colour Wheel
• Luminosity and Tints & Shades
• Colour should be Fun
• Colour Calibration :: Before You Panic I
• Colour Calibration :: Before You Panic II