There is an option for the guild tag colours to be changed. (The guild tag is the small colour block in front of the name of all the guild members). The default is green.
If any of the guild leaders would like to contact me to have the colour of your guild tag changed, please feel free. Please ensure that you check with your guild members that they are happy with the colour you choose, or perhaps take a poll.
Please supply me with a general colour (red, orange, green, etc), and I will see what I can do.