Speckle is one of the most important issues to develop laser projectors. In this paper, it is shown that the effect of wavelength diversity and angular diversity on speckle reduction is not independent, and that the effect of wavelength also depends on incidence and observation angles on screen. The speckle reduction effect by wavelength diversity is smaller when the angular diversity is larger. The results would be helpful to determine the wavelength selection in laser projectors to suppress speckle effectively.