Thanks jaybiz3,
Yes that's correct, you can enter any basic colour using the commandline. Also, you can set it to /traycolour=auto and the colour will change depending on how you're doing:
What do the different colours in the tray mean?
The colours are as follows:
Green: you're within your limit and aren't on track to go over it before the end of the month.
Orange: you're within your limit but you're going to get close to the limit by the end of the month (90% or more)
Red: you're over your limit already, or you will be by the end of the month.
For Windows Vista and above, the tray colour is set to white the whole time, but it can be changed back to automatic by specifying /traycolour=auto on the commandline.