I got a few pens today and tested each color (it was two sets, hence some of the odd colors) on some spare keycaps I have:
I'm surprised at how well it came out. I can't be very delicate with the pens since the tips are fairly large, but I can distinguish the keys just fine. Even the black is visible in a well let room. I was surprised how well the gold and silver worked, the other colors are almost all multiple strokes, but the gold and silver are single strokes.
The only color I would def not do is purple, it's too dark and I may just not like purple
This is my current setup
I think I will do some orange, green, white. I *might* do some black, silver and gold if I can come up with good uses (thumb clusters?) but I don't want to make too much of a rainbow