Rather than starting with Primary, Secondary and Tertiary Colours, let’s start here with ‘Web Safe Colours’, as they form the basis of the Coloured Trees presented in The Tree in Colour section:

As explained in numerous sites across the web, the 216 colours with 6-figure hexadecimal or ‘hex’ codes were determined some years ago to be the basic building blocks of commonly recognised colours that could be reproduced faithfully on the screen by the leading browsers around 2007.  Wikipedia’s article on Web colours will probably tell you more than you want to know about some of the technical aspects of this, however perhaps not as much as you need to know to understand how colour is used or has been translated for the Tree of Life.  An additional complication is that it seems the concept of ‘websafe’ colours is now more than a bit obsolete. This excellent article called Death of the Websafe Colour Palette? provides further insight but perhaps no more clarity on arriving at agreed colours, which is one of the fundamental issues highlighted in this small investigation.

I’d originally thought of summarising some colour basics here like Subtractive and Additive Colours; RGB vs CMYK, Colours of Light vs. Pigments, etc.  However, after compiling the Colour Resources page, I realised that all that basic info and more is available there, and is explained much better than I could do.  In this regard, I’m hoping a real artist or web designer will comment on other colour issues and resources i’ve surely missed — and that I’ll discover someone has already published a guide to astral colours, which may or may not translate info hex codes and pigments.