Article 8 states:
Colours and patterns
8.01If a playing attire item comprises three or more colours, one must be clearlydominant on the surface of that item. For hooped, banded, striped or checkeditems (i.e. with two dominant colours), any use of a third colour must notdominate or affect the distinctiveness of the shirt or socks.
8.02The dominant colour (or colours in the case of hooped, banded, striped orchecked items) must be equally visible on the back and front of each item ofplaying attire, excluding the number zone on the back of shirts(see Paragraph 7.01(b)).
8.03The colours used to create a hooped, banded, striped or checked shirt on thefront must both be clearly visible on the back of the playing attire if the samepattern (e.g. stripes) is not used on the back of the playing shirt.
-This also has some pictures of what is and isn't acceptable.
Article 53 also matters:
53.03The contrast between shirt colours in the number zone must be less than or equalto a Delta L of 25. Colour combinations with a Delta L difference value greaterthan 25 require a single-colour patch on the back of the shirt to serve as thenumber zone.