What causes reflection clouds to appear blue to an observer?

Fine dust particles preferentially scatter the blue wavelengths of light from nearby stars more efficiently.

Reflection clouds are distinct because they neither generate their own light through emission nor are they dense enough to block light entirely; instead, they scatter light originating from nearby stars. This scattering process is size-dependent. The fine dust particles within these clouds interact with visible light by scattering shorter, bluer wavelengths much more effectively than longer, redder wavelengths. This physical principle is known as Rayleigh scattering, which is the same mechanism that makes Earth's daytime sky appear blue. Consequently, the light that reaches the observer's eye is dominated by these scattered blue components.