checking if a channel accepts coloured text is a bit hard because the channel mode isn't the same for all server software. sometimes it's +C, +c or +S or something totally different.
you can always notice by the error reply you get if the message was indeed blocked...
next to that: mIRC only supports the normal 16 colours available for IRC. as you can see when using mIRC yourself, it sends the colour in the format of ascii code 3 followed by a number (0-15).
the actual display colour is not directly defined since users can change that in their own client but check out the defaults mIRC has.
it's best to stick with those 16 colours because you will be sure almost all clients will be able to understand that.
