ANSI Z89.1-2003 Appendix A4 states that caution should be exercised when painting hard hat shells since some paints and thinners may attack and damage the shell and reduce protection levels. The hard hat manufacturer should be consulted before painting. This specifically refers to industrial hard hats but the intent is applicable to bike helmets.
In addition paint or stickers may interfere with the ability to properly inspect a helmet for cracks or other defects which may reduce its ability to protect the wearer.
From the Bicycle Helmet Safety Institute: https://helmets.org/paint.htm