there are 3 planes, each plane has two particle hair systems. the particle hairs that on the same plane has the same material. in blender 2.83 LTS. the color display as intended. since 2.90, the particle hair looks different. when both hair system are active, the color is mix with other hair particle color of different planes, if disable either one of the hair system that on the same plane, the color return to normal.
as you can see in the picture below, I set the material to mix of primary color red green and blue with secondary grey color. in 2.83 it looks right, but in 2.90 and above, the blue and green materials also mix with red somehow.
here is a link to the blend file: https://drive.google.com/file/d/1Ai8InP7sNN0hHDHC-sD8gvr5ZL9BCEz9/view?usp=sharing