If you look at the study you can see that they also had a treatment of black stripes on black cows to control for just that:
The cows were assigned to treatments using a 3 × 3 Latin-square design. The treatments consisted of black-and-white painted stripes (B&W), black painted stripes (B), and no stripes (CONT) as a control (Fig 1).
Thanks for the response. 🙂 I indeed hadn't look into the paper as there was no link in the original post, but I have done so now and it seem, you are right. Thanks! 👍
Well, it is certainly a sign of good critical thinking skills to ask a question like this! And thanks to you I actually had a look at the study itself :)