You can paint the heat spreader, but there are a few things you'll need to take into consideration.

First, a coat of paint will probably make them less effective at removing heat from the memory sticks.

Second, you should be extra careful to avoid getting any paint on the electronics or the connector. Ideally you should remove the heat spreader before painting it. If you can't remove the heat spreader from the memory to paint it, then you should cover any exposed circuit board before painting. Some paints are conductive, and that can wreck havoc on electronics. You also need to be careful not to get paint on the electrical contacts where the RAM is inserted to the motherboard.

Honestly, I wouldn't recommend painting the sticks because aside from appearance you risk damaging the memory at worst and degrading it's thermal performance at best. That being said, it's definitely possible to do.