If you mean typing in a colour all the time... You can't unless you write a script to do it. In the colours options you can set your own text to appear in another colour but mIRC always sends plain text to the IRC server unless you either include the script I mentioned or add colour codes each time you type.
You can create a permanent quit msg in your mIRC , so you just need to type it in once. Go to file/options/irc/messages and type in your quit msg there.