Black cats with the recessive "agouti" gene appear reddish or cinnamon in sunlight.
Sunlight triggers increased pheomelanin production in agouti black cats, creating a warm brownish hue.
Full sunlight alters ratios of eumelanin & pheomelanin in agouti black cats, making reddish tones visible.
Genetically, cats that shift color in the sun are not "true" black cats. They have a modified black coat.
To our eyes, the coats seem to change color, but it's just a reveal of underlying pheomelanin in the fur.
The cat's actual hair color remains genetically unchanged. Sunlight simply makes hidden red/brown hues perceptible.
The coat color change can be quite dramatic on some cats, going from black to cinnamon or chestnut.