Under DDA regs, the route number and terminal point should always by shown on the front.  If the service is registered with the TC and it has a route number, then under the rules, it must be shown at all times.  If they just register the service as Rainbow (even thou their buses are not painted in rainbow colours ) for example, then this needs to be shown at all times instead of the route number.

There is nothing in the regs about colour, so whilst some colours do not look good to see, they are fine with that.

As you say, its difficult to argue a case, however if the things I have mentioned above are not constantly shown, they are breaking the PSVAR rules.