PrusaSlicer multicolor, unwanted color changes on infill  

PrusaSlicer multicolor, unwanted color changes on infill

I just upgraded my Core One to add an MMU3 (not easy!) I have successfully used the Prusa slicer to create and print new multicolor designs which include multiple color changes in a single layer. But now some, but not all, of my designs are getting sliced with unwanted color changes in the internal infill layers. This normally wouldn't be a problem, but since my outer layers are white, these color changes are visible through the top and bottom layers. How do I edit them to be the color I want? 

My original files are STL. I used Split to Parts and the Multimaterial painting tool to color a set of simple tags. The top and bottom layer colors are correct, but see in the attached photo that the bottom tag (and only the bottom tag!) has a rhomboid shaped artifact where both red and blue infill has appeared. (All three tags will have  text on the topmost layer, the lower tag's top text is wider which seems to require this odd rhomboid infill. The rhomboid isn't a problem, but the red and blue color is a problem.)

I did find the instructions to change Print Settings > Multiple Extruders > Infill extruder and I set it to extruder #3 (white) but this had no effect on the slicer output. 

I also tried to manually find the STL "part" for the rhomboid in the table which shows the extruder used for each part, but even after changing all the parts which used Red to another Extruder color, I could not find and change that rhomboid infill.