After much hemming and hawing (say what?!) I have finally settled on button colors. I think I have commitment issues. I'm a perfectionist, and being a perfectionist means over-analyzing and thinking about stuff, and in this case, I'm still trying to decide on cabinet artwork, but I did finally settle on some decisions (phew!):
I was originally going to go with blue buttons (perhaps even 2 shades of blue). But the widest variety of colors are only available for leaf-switch. My control panel is way too cramped for those leaf-switch holders, so I'm going with Happ pushbuttons.
So then i was looking at the blue and purple that they have, and then I thought about what actual arcade games usually used for button color choices (why not copy what the original designers felt was best?).
What I found was that usually they use different colors for different buttons, because it helps the player remember which does what. So that settled it. I'm going with multiple colors and will compensate for the "rainbow" effect through a good CP art design.
My final button colors will be red, green, blue and yellow, placed in a configuration that matches the japanese Super Famicom controllers. There. If ever I wanna play SNES emulators on this cab, I'll be set.

(Uh oh... the american SNES opted for a two-tone button color scheme... HMMMM there goes my mind again...)
~Ray
PS: I had the same "can't settle on a plan" problem for my cab color scheme. As nice as the red and yellow is, truth is I have already purchased blue t-molding. Since I'm in Canada, ordering t-molding isn't the cheapest thing, so I will stick to my original blue T and white sides plan.