
Brick colors aren't actually rendered through their color ID, they're already implemented as full RGB.
So let's say that, instead of having a fixed colorset, the server and client have a "virtual" colorset that is used to send colors of groups of bricks. Barring mass spam, which is going to take up bandwidth anyway, the vast majority of bricks in the same area will tend to be in the same group of 64 colors or so.
As a result, any old builds take up the same amount of network bandwidth - the colorset already has to be sent anyway - and any hosts that don't like the idea of anyone using any color can still trivially disable it.
There are other reasons that full RGB would be undesirable, but using network concerns as one doesn't really stand up on its own.