On a serious note...what you choose depends on the look you have envisioned for the end result. Colors can be grouped into catagories like 'cool' colors, white, light blue, silver, (think ice, snow ect.) or 'warm' colors like reds, yellows, oranges and so on, ( think fire). Then they are used as complimentary, meaning the colors used compliment each other like red and orange, or green and blue, or they contrast like black and white. Using colors that fit within these parameters should result in a half decent job.
Then...take as a fer instance...you paint the car with a base coat of emerald green, you could use blue flipflop. or fine gold metalflake in the clearcoat, ( green and blue are complimentary and gold is a contrast)
Generally, there are certain colors in certain shades that do not work well together, pea green and black, tomato red and leaf green etc.
Probably the best way to see what will look good is to fire up a paint program on the 'puter, find a pic of a similar car and experiment painting it there first.