White is best. I painted my last darkroom white and will do so when I build out the current one. The only light present should be safe. The wall color does affect how much safelight you can use but makes it easy to bounce safelights off the walls (and ceiling.) You should test and adjust the intensity in any case. You might have to dim your safelights a bit with white walls but you'll still have more even illumination at the same level.