You can use the built-in features of mIRC to color the nicknames of people who have been idle (i.e., haven't said anything in that channel for some preset time), but I'm not sure if that is exactly what you want. If it is, let me know and I'll show you how.
If you want it based on being marked as "away", it is a bit more complicated. When someone marks themself as away, your IRC client doesn't actually know about it unless you attempt to send a message to that user or perform a /WHOIS on the user or /WHO on the user/channel. So basically the most efficient way is to /WHO the channel on a short interval to keep your away list updated. This would require a small amount of scripting. Is this in line with what you're wanting to do?
