More sunlight bounces off the light colored rock. More sunlight is absorbed by the dark colored rock. More of the sun's energy is captured by the dark rock, so it feels warmer. An object may both reflect some light, and absorb some light at the same time. If it reflects certain colors from white light, and absorbs other colors, the object will take on a color of its own.
Paint changes the surface of objects.
Mixing paint combines colors physically into one newly formed color. Only the light bouncing off the mixed paint is what we see. So red paint will absorb some light out of the light that shines in it. Green paint will also absorb some of the light that shines on it. When red and green paint is mixed, the mixture captures all of the blue, and part of both the red and green from the reflected light.