I've recently regretted applying colour to to some mods in my RimPy modlist and figured out how to reset mods to their default 'colourless' state.

1: In the top left corner of the RimPy window is a button called 'Settings'. Click it.

2: In the resultning window is a file called 'config.ini'. Open it with notepad or a similar program.

3: Under the category [Colors] is a list of mods followed by their colours in hexadecimal, ordered (as far as I can determine) by the date they were coloured, with the most recent at the bottom.
3b: If the mod isn't listed here, it is the non-coloured default.

4: Determine which mods you you want to 'uncolour' and delete them from this list.
4b: You can do this using the mod's ID (ex: the ID is 1847679158 for 'RimPy Mod Manager Database')
OR
4c: You can use the colour picker to find the hexadecimal code for the relevant colour and erase all entries using that colour
OR
4d: You should be able to recolour all the mods you want to reset to an arbitrary colour, exit RimPy and delete all the lines at the bottom of config.ini that shares that colour code.

5: Exit RimPy if you haven't already.

6: Save and close the config.ini file.

7: Start RimPy, the mods whose lines you deleted will now be the default colourless.