Liam
Post #107 by Matt quotes the rule in full, but I would say that the common sense application of this is if you have a cap, jersey, hopper or boots that is entirely or mostly yellow, you cannot use them.
If some of your kit has a bit of yellow in it, you are good to go.
As has been said, it is a rule designed to help the player, so that the colour of your kit does not get mistaken for a hit and get you pulled.
If anyone has some kit that they are unsure about using because of the colour, post up a pic and I will link Ains & Jabba (Head Ref) to it, and you can get the word from the top.
Hope that helps.