How do they know the paint didn't do it?

There were 3 groups of black cows: an unpainted control group, a black stripe group painted with black stripes (not very visible because the cows were already black), and a black and white painted group. The control group had similar results to the black stripe group, which suggests that the black paint alone didn't do anything.

So further research could be to compare to an all black painted group and an all white painted group, with no unpainted fur, as well. If it's the pattern, then one would expect the totally painted cattle of either paint color would see similar results as unpainted.