I polypainted specular (white, grey) color on my model but I was painting on skinshader4 material, I switched to flat color to see better what I painted and I got this strange thing instead of flat color with my polypainted greys.
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you were painting material before as well, so you need to clean that off the model.
Select the flat color shader. Turn on M. Turn off RGB.
color.fill object.
Should be all set.