How do I group categories using colours?

Your question seems to confuse two things, namely OpenStreetMap data and map styles. Mappers and rendering style designers have separate tasks:

• Mappers collect data about reality. For example, they add the information "here is a building made from clay" to the database.
• Rendering style designers define what aspects influence the appearance of the map in which way. For example, they define "buildings made from clay should appear as a light brown shape".

So if you want a new class of information to appear on the map, you also need to do two things:

• Check whether there is already a documented way of mapping this on the wiki or elsewhere. If there isn't, invent new tags to represent the information and use them on objects in the database (preferably after discussing your tagging ideas with other mappers).
• Convince style designers to take this into account when rendering maps, or render your own maps.

This probably doesn't answer your question entirely, but I hope it helps you to understand what exactly it is that you want to do and to ask a more specific question that will result in better answers.

No, the renderer and its stylesheet define what colors to use for which object (that is, Mapnik for the main page). If you want to change those colors, you have to render your own maps.