The Default should be the Automatic one for Incoming Messages.
This will display the Message as sent by the Buddy (AIM, Jabber or Yahoo) or clear/white for iMessages form an iOS device as this sends no colour.
You can change it to select a colour and font but then all people look the same.
iMac 2.5Ghz 5i 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad