I would use bright primary and secondary colors - blue, red, green, yellow, purple, orange. Think about a small box of crayons.
My mother has made 2 afghans for my son's future children. She is afraid she may not be able to crochet by the time he has children or she may not even be here. He said he liked bright colors over soft pastels. My mother had originally thought soft green or yellow - like what was done in the days before sonograms.