I'm going to assume because the IRC protocol involves colons, this might be a parsing error as Chrono mentioned.
I've gone through testing and it's not actually a problem with the receiving part, but instead the sending part.
I was able to see the beginning colon when sending a private message through mIRC.
The sending part was the problem, in RTBIC_sendSessionMessage
IRC doesn't seem to care whether or not you put the required : before your message, seeing how both
RTBIC_SC.sendLine("PRIVMSG Spy2 lolstuff"); and
RTBIC_SC.sendLine("PRIVMSG Spy2 :lolstuff");gave the same result, "lolstuff", but
RTBIC_SC.sendLine("PRIVMSG Spy2 ::lolstuff");has shown the initial colon. ":lolstuff"
RTB doesn't send this : for normal PM messages, seen here:
RTBIC_SC.sendLine("PRIVMSG "@%sess.username SPC %message);where %message should have a colon behind it.