I vote have coloured text by default, as that's easier to see who said what. However (and I think Z suggested this) then, if this feature is included, it should be toggleable between name and text - by default a user would see all the text coloured, but if they hit a switch then they see those colours on the names rather than the text.
Of course, if you're going to do that you might as well make those options independent - either fully coloured, or just names coloured, or just text coloured, or no colour at all.