Hm never actually tried, but i looked at Manual now and this says:
0 - Black
1 - Red
2 - Light green
3 - Yellow
4 - Blue
5 - Purple
6 - Light blue
7 - White
8 - Dark green (default
Applies only for dedicated server. For non-dedicated, see above. Open your server's *.cfg file in notepad and edit the host name entry. See above for the colour codes and precede the number with a ^ character. Eg. /host=^7Ho^4st Na^7me - would make the host name have colours of the Finnish flag.