In an old version of this article, it described how you could change your MSN chat colour to a custom colour using the registry.

A much better idea is simply to install Messenger Plus!, which gives you a dialog box in MSN itself to change the text colour.

However, the relevant registry entry is still the same, which is: JKEY_CURRENT_USER\Software\Microsoft\MessengerService#IM Color And the data there is stored as hexadecimal red/green/blue/zero.