mIRC currently has sixteen colours, going from 0 to 15, taking up two characters. My suggestion is to introduce 6-bit colour.
A 6-bit RGB palette has 64 colours. I say we take numbers 20 through 83 and map them to these 64 colours.
My guess is there might be IRC clients which map numbers 16-99 to the original sixteen colours currently. So, for the purpose of backwards compatibility, I say with numbers 20 through 83, we map the respective shades (as close as possible) in an order most fitting. For example, since red is 4, then 20, 36, 52, and 68 would be shades of red from that 6-bit RGB palette.