If you would like to modify the color of all buttons your client will see, you can navigate to "My Account", "Account Settings", "My Preferences", and click on the "Edit" button.
The color can be assigned by color #hex value or by using the tone box.
Do you have a specific color you want, but you don't have the hex value?
Here are some resources that you can use to obtain it.
- Website Service: http://html-color-codes.info/colors-from-image/
- OSX: Use the built in app called, "DigitialColor Meter"
- Windows: It is recommended that you use a website service like the one recommended above. Alternatively, searching "color picker for windows" will yield several results for freeware (proceed with caution).