The two stripes down the middle of the 78Comp body family don’t seem to have a paint slot. Applying a paint to all body/trim slots doesn’t paint the stripes. The only slot that still shows the default red color is the wing mirror slot, which is greyed out.
the stripes color slot is in the trim tab
Applying the paint to all trim slots worked. The actual trim slot works too. I guess the issue now is that the slot initially appears to be set to the plastic material, but it remains the default metallic red on the car itself.