So we were thinking about painting our robot purple. Would this be illegal because there are purple cubes on the field?

Just be careful they aren’t the same purple. So long it is obvious enough you shouldn’t be given a ton of trouble.

Here is the official GDC response -

It is impossible to issue a blanket ruling that would cover all hypothetical decorative styles and color shades. We would advise Teams and Head Referees to refer to G3 (“Use common sense”) when determining whether a given decoration is trackable way by another Team’s Vision Sensor (under realistic, Match Affecting circumstances). The intent of G12 is not to prohibit all teams with green, orange, purple, black, gray, blue, or red themes from decorating their Robot; it is to protect (within a reasonable extent) Teams attempting to take their VRC experience to the next level through Vision Sensor programming.

One way to think about decorative color choices is to view them the same way you would a typical non-functional decoration, such as a giant decal. By itself, a giant decal is considered a nonfunctional decoration. However, if used to hold metal parts together or to hold game objects, it has become functional, and is no longer legal.

That being said, just as teams are responsible for the decorations on their own robots, teams utilizing the Vision Sensor should be conscious of the possibility for inadvertent or incidental visual interference. It will be up to the inspector and Head Referee to determine if a given Robot’s decoration or design acts as a “Vision Sensor distraction”; to assist in reducing the frequency of these judgment calls, we would advise Vision Sensor Teams to take advantage of the engineering design process to investigate programming solutions that mitigate the impact of undesirable conditions.

To summarize more colloquially / bluntly: yes you can paint your robot green, just don’t hold up a giant green sign if playing against a team that you know tracks green cubes, and if you’re going to use a Vision Sensor, remember that there may be some green things in the arena that you have to account for.