You can have the almost every colour in mIRC, the limitation however is on the quantity, you can only use 16 colours at anyone time.
i.e.
//echo 2 This is dark blue.
//color 2 $rgb(255,179,236)
Will firstly, echo a dark blue line, then it will replace the darkblue in your palette with RGB value '255/179/236' (a pretty pink).
Use, /color -r 2 to reset it back to blue.
Eamonn.
Eamonn.