AFAIK mIRC doesn't have the infrastructure to modify parts of an existing line in the channel buffer. Only the "line colour" itself can be modified.
Even if there was infrastructure for this in place, there would be tons of potential incompatibilities with existing scripts, as well as handing pre-existing colour codes. For instance, if I wrap my urls in a colour code, which colour wins, mine or mIRC's? What happens if mIRC's visited colour equals the background of the given text?
Tons of edge cases here, not sure it will go over well.