You'll have to wait the expert rank to get the orange colour. You'll need to reach 500 messages for that (but please, don't spam just to get that title).
But as Danny said it's a preset colour, you can't change it.
The moderators and admins have their own colour, and TtTOtW and me have the special "MOTY" (member of the year) title, which comes with its own colour, but that's all.