You may have noticed when setting up membergroups you can set a colour, now yes you can put a colour name in the box, but my experience usually shows me that the internet specification for colour and my interpretations of colour are different. So here are some colour codes as hex values which may help you more.
Admin Panel -> Edit Membergropus -> Modify (click this on the groups colour you wish to modify)
You will get a new screen and it will have a box that says "Color in online list:" across from that you can set a colour from the list below, always remember to put the "#" at the start (without quotes) example "#00CC00" goes in the box for green.
#00CC00 - Green
#0000FF - Blue
#FF00FF - Pink
#660066 - Purple
#FFFF00 - Yellow
#FF3300 - Orange
#FF0000 - Red
Other colours - http://html-color-codes.com/