This is actually a basic usability issue: it should never have been the case that users with one color scheme cannot read text from those with a different color scheme, eg. if you have your background set to white and someone else has it set to black, and you then say "^K1Hello", it will be invisible to the other user. Communicated text should always be visible.
A good example of this is the server MOTD for irc.paraphysics.org, where they use a white foreground color for some of the text which makes it invisible in the default mIRC color scheme.
I think it might be possible to resolve this issue without removing the feature: if only a foreground color is being used, mIRC will check for duplicate foreground/background colors and fix the foreground color if necessary. However if both a foreground and background color are used, no check is performed.
Does that make sense/sound reasonable?