As @bark mentions, each voice it’s own colour, or for different signal types.
When I started with my modular I tried to colour code, but you never have enough of the right colour, and stackable cables (which are coloured by length) are very convenient.
Now that the modular and cable pile are much bigger, being able to grab the right length is very useful. I find that the stackables get used first mainly because I can find the right length easily.
I’ve been think about pre-patching bits of my modular and seeing how that changes how I interact with it. I’d probably try to do the pre-patch cables in a single colour, partly so I know which ones they are (but mainly for the aesthetics).