Not ready to paint 84 yet but want to start planning ahead. My concern regards color change between body panels and bumpers when flex additive is added. I have no experience here and don't really know what to expect. Have seen many cars at shows with very large degree of color change. How can this effect be minimized? Do you have to "mix" your own bumper paint? If so, since mine is dark bronze, do the bumpers get darker or lighter with the flex additive in it?