First off, I know how to give my AI custom colours/skins, and I know about naming a new colour set to have it applied to the AI.

What I want to know is if it is possible to modify the game's stock preset colours for the AI? Is there a file somewhere that can be hex edited to use a custom skin and/or custom colours?

Don't get the question

Thats not what I meant, wheel4hummer. I know about all of that. What I want to do is replace the existing pre-set colours with ones that I choose. Every car has a few colour presets that can't be changed in game (represented by coloured blocks). These are applied to the AI by default when "AI use player colours" is set to 'off.' I want to change these presets. Having presets with "AI 1" "AI 2" etc as the name takes up space in the menu screen and get in the way. Plus, they are only applied to the AI with that name. The presets are not on an AI-specific basis, they are just applied in sequential order to the AI as they are added. That is why I want to change them; to easily have custom AI paintjobs of my choice without taking up menu space.

I know what you meant, it was an easy question, people should learn to read.

Anyway, I think you cannot do that as they seem hardcoded into LFS.exe. There are no default .col files that represent the standard colors.
Otherwise it would be a simple case of renaming...