A:
If they are two different brands of paint, it is a possibility for the paint to become weakened, or for the luster to be uneven (if one paint is a matte, while one is an eggshell finish, etc.)
Another problem you face when mixing paints yourself is unevenness of color. While to your eye it may appear to be completely and evenly mixed, when you combine paints it is extremely difficult to make it 100% even, depending on the level of difference in the two shades.
Is this answer helpful?
3