I'm just here to introduce you to the registry key, and I hope someone will make a small tool modifying this registry setting soon
(If there is still no one doing so after 24 hours, I'll make one myself anyway
)
The registry key is at HKCU\Software\Microsoft\MSNMessenger\PerPassportSettings\(Your passport ID) ->
InkColor
Just modify it as you did for message text color before. It's a Binary key by now and the format is like RR GG BB 00, such as setting it
ff 80 c0 00 creates a pink-like color. (Of course, you need to re-open all your conversation windows so it takes effects)