CP Layout and colours.

So I'm getting to work on my CP for a mame cab. I sort of want to go with a neo geo theme but want to use 6 buttons. Id like to do the red,yellow green and blue buttons, looks nice and its easy to explain to your friends what buttons do what. But of course that leaves two more buttons. No sure what colour to make them.
Layout wise I going with this (6 buttons):
http://www.slagcoin.com/joystick/layout/sega2_s.png

Was thinking of going with this layout, but thought that would put the controls all over the place for street fighter:


So going to go with this:



So do you think I should leave those two other buttons black?