In Editor Preview only (not in play mode, not in build), the particle system’s randomization is incorrect. With the following settings, the particle’s Z position within the system partially determines its color - that is, particles farther along the +Z axis are far more likely to be gray, and vice-versa. It’s possible other settings are affected by this as well. It seems like the system uses a static number as its random seed or something. Raising the emission rate high enough hides the effect.
Is this a report-worthy bug? I wasn’t able to find any info on the net about it.
This first image shows an average image, the right half of the system being significantly more blue than the left:
Here is the full setup, try it at home:
