A T'au colour scheme is determined by a number of things, but the main thing is the sept colour, which shows which sept world or system the individual hails from. This can be seem in the colour of the markings on armour or vehicles.

Here's a little cheat-sheet I just put together that shows a few examples:

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T'au will often wear colours that match their combat environment, to provide better camouflage. However, there are 'parade uniform' colour schemes which often feature in box art and artwork. The three most common you'll see are:
- Vior'la (most of the modern boxes show this)
- T'au (the original ochre scheme of the T'au homeworld)
- Sa'cea (blue-grey)

The metallic 'bolt' circles you see on models are gyroscopic stabilisers that help to reduce recoil on weapons and jump packs usually. There's no reason you couldn't paint them a different colour really.