The silver flakes are great with blue, white, black or silver. Gold flakes are good with yellows, gold or browns. There's also a pearl flake, but that's tough to match since it sparkles with pinks and aqua as well as white. I think I have 5 or 6 different colors of flakes, so I have good choices.
There's also the blue flake which pretty much only goes with blues. Same for the burgundy. (that one's beautiful, and it's going on my next paint job with candy apple paint)
I put the bottle of flakes next to the paint bottles till it hits me with a nice combination.
If you're not quite sure, just do it on a scrap piece to check it. Same thing though, do it in reverse for the proper effect, and allow it to dry completely.
I did an entire body, and was very dissappointed in the effect. 2 hours later, when everything was dry, it was absolutely beautiful.