When trees are young, the bark is tender and can get damaged by direct sunlight (basically sunburned). Once the bark is damaged insects and diseases can infect the tree more easily. Paint protects the tree like sunblock, reflecting sunlight from the trunk so the bark isn’t damaged.

White is the most reflective color, so it’s best for preventing sun scalding. Other colors will absorb more heat/UV from sunlight so they’re not as good protection… particularly dark colors can be worse than nothing since they’ll heat up more than natural bark.