First let me say that no safelight is completely safe. They will all cause fog eventually. The choice between red or amber is determined by the material being used. Use a safelight filter recommended by the paper manufacturer. What counts is the total amount of light in the darkroom rather than any mixture of red and amber.

Amber is preferred for most papers since it provides the best visibility. The human eye has a problem with red light and visual acuity is poorer than with shorter wavelengths.