I used a set of
Sharpie Neon markers. They're pretty easy to come by; I've seen them at most craft stores. You probably saw them already because you mentioned the five-color limitation, and that's what these have. But you can make other colors, like brown and purple, by combining those five colors. Give it a test on some paper and you'll see what you can do.
I found that the Sharpies were too intense for flesh tones. There I used two markers (040 and 054) from the
Stablio Neon Fine Points set, blended together to achieve the range of shading you see. I believe I got those from Blick Art Supply, but I'd guess other art supply outlets would have them too.
For the black I used a standard black Sharpie, and to get quicker coverage on the backs I used a very wide point Sharpie. I went over the backs several times to get them black enough. Be careful though. A little too much black, or holding too long on one spot, and you might get a bit of leak through to the faces.
There's a white trim on the back of the cards which was applied with a white paint pen (once again, craft or art store). The pigment ink covers the black, but doesn't show up well in black light. I don't care much one way or the other, but I like the way it looks in white light.
I went through lots of markers. I think I bought at least two sets of the Sharpie Neons, maybe more. I bought a number of black Sharpies too. Coloring an entire deck really sucks up the ink, and the deck maintains an ink odor as well. A friend of mine didn't like all that ink smell, but I kind of do. But hey, I'm weird.
If you hate coloring things in, you won't like this project because it's LOTS of work. But if you enjoy it, you might find this LOTS of fun.