They might mean the paint itself is abuse, which I think most vegans would agree with, but the more solid part is the abuse around owning either a commercial farm cow, or a research and testing cow.
It is just a picture but its hard to assume this cow came in from the grassland asking for help with mosquitos, and we nice humans obliged.
It sort of gets into reducing the animals free will. It might not mind the paint but it also might not like it, and it can be hard to tell. There is also a good chance its toxic or causes some affect unintentionally.
Usually the thing I'll ask, is who is benefiting from this action? If its not the animal, it probably shouldnt be done.
I think a good counter example is collars. My dog loves his collar because it means he's going outside. It keeps him safe and others safe too. The collar is a good thing.
Edit to add: I am by no means a great reference on veganism, just doing my best!
Thanks, yeah that is an interesting point. I suppose any sort of experimentation becomes a bit more ethically dicey when the scientist can't have informed consent from the subject.
Theres also some debate over how useful it is to experiment or test products on animals from a scientific standpoint. I do think scientists understand more than most how gradual change can be though so maybe thats why its not as loud a conversation from them.