In the past, on previous websites the colour of the group ranks have been decided on the colour scheme of the theme. Normally I just stick to the normal colours, Admins & Mods, although I have sometimes used different colours such as, Admins & Mods.
Although I do normally prefer the admins colour being red, it stands out a little more than any other colour and is just generally known by everyone who uses forums that if the colour is red, they are more than likely an admin.