Yes I agree that mIRC contains that feature BUT if you have a black background and lets say normal text in white and somebody uses foreground color black in his text, you will see nothing. Try this line of text on a black background without and with the color replacer installed:
1This is unreadable on a black background, 2 and this is difficult to read
By the way, I have to update the snippet for on ACTION and on NOTICE, I forgot about it :S