Disclaimer: Fruits Basket is the property of Natsuki Takaya. The following is a work of fanfiction loosely based on that property. This work is unofficial.
The expression Kyo had always heard was "seeing red." People would get angry and "see red."
He never understood it. When Kyo got angry—truly angry—he saw only black and white; all color faded to gray, and everything sharpened.
As a child, he once had a conversation about it with Hatsuharu on the playground: after making the mandatory jokes about "seeing only black and white," Haru pointed out that when Kyo was in a rage (usually at Haru, but sometimes at the Rat they both despised) his eyes changed color, and his pupils changed shape.
Kyo was horrified and his right hand fidgeted with his left wrist, making sure the bracelet he hated so much was still there.
It was only much later that he remembered that transforming into a cat had an identical effect on his vision: he could see in the dark—and the world became a cold, joyless place devoid of color.
Kyo was very sick when it happened; it wasn't as if anyone was ever going to hug him.
After the color finally returned to his vision, Kyo told himself that he wouldn't let anyone hug him ever again.
Author's note: further research (conducted after this chapter was written) indicates that cats can differentiate colors on the blue/violet end of the visible spectrum, but have difficulties with red-through-green.