Never heard green and yellow box... where did you heard these terms? Any ways Graybox testing assume that you have knowledge of the system, how it is designed, what code is lying there and you design/execute your test cases based on this knowledge. More information about this can be found at http://testinggeek.com/graybox.asp
As far as colors are concern, there are only blackbox, whitebox and gray box testing. Will be interesting to know other colors.