And still many channels allow colors.

I don't know why this should open for more flood. If they wanna flood for urls they already are, and the colors are not making it more interesting for the flooders.

There are channels that doesn't allow public away. It's kickban.

I don't know how mIRC colors work, but if mIRC got 54 colors then the new colors would turn out black for other clients. That is a way to make it.

This color issue isn't that much of hard work to add for mIRC?