As it relates to their physiology?
Or maybe their genes?
It’s puzzling the heck out of me as I watch one horse go from light to dark while the other horse goes from dark to light.
Peppermint, the pony, is at her darkest brown in the summer. Right now, she’s light khaki brown fuzz ball.
Comet, with similar paint markings, is at her darkest in the winter. Right now, her otherwise brownish coat is jet black.
The two coat hues met somewhere in June or July.
That’s because the long and sunny days bleached out Comet’s coat.
At about the same time, Pep had shed out her lighter-colored wooly look to reveal a lovely, dark chocolate tone. She lightened up significantly less under the same summer sun.