If you turn your piece, is the "dark" side always the same? If yes, it's about light/shadow settings . If your part is made with symmetry, sometimes there are bugs with the colors, especially when you change the color of the faces after creating the symmetry function.
Maybe the light bulb of a spot that has failed. A little humor with these difficult times. I actually think like my colleagues that it's a problem of light/shading adjustment and that you just have to activate a spotlight on the other side or deactivate the one that is active.