Synthesis of ZnO by wet chemical method and reducing particle size by ball milling
I had a parallel experience and noted it in my thesis, 20 years ago.

Was working on an oxide ternary phase diagram (not wet chemistry, traditional shake and bake).  There were some conducting phases (dark) and some insulating ones (white).  I had a ZnO pellet (post firing) and I ground it under acetone (just mortar and pestle).  There was a definite color change to yellow.  

I had a lot of other results and never ran this issue to ground.  I did note it and speculated that it had to do with the grain interiors having some conductivity (e.g. from oxygen defects or from impurities) and thus a different color, with the grain exteriors more insulating and white (not having the defects).  This would explain the color change.  There have been papers written (more than the 20 years ago) about differences in defects at grain boundaries and interiors of ZnO.

Very much a speculation at the time and never looked into it more.  But very interesting to see you have a parallel experience.  Maybe you will do more than I did and run it to ground?

P.s.  Consider the appearance of In2O3 and Ga2O3.  Indium oxide out of the jar is yellow and has semiconducting properties, from oxygen defects or the like.  Gallium oxide is much more insulating and is bright white.  Perhaps your ZnO grain interiors are a little In2O3 ish?  After all, both materials are dopable (despite the large band gap) and are TCO materials.  Ga2O3 on the other hand is extremely insulating and undopable, with a wider band gap.