We only know roses are red and sky is blue,but thanks to specialized cells in our eyes called “cones”.
When light hits an object , the object absorbs some of the light and reflects the rest of it. The wavelengths that reflected or absorbed depends on the properties of the object.
Cones are one type of photoreceptor, the tiny cells in the retina that respond to light. Most of us have 6 to 7 million cones, and almost all of them are concentrated on a 0.3 millimeter spot on the retina called the fovea centralis.
Not all of these cones are alike. About 64 percent of them respond most strongly to red light, while about a third are set off the most by green light. Another 2 percent respond strongest to blue light.
When light from the object hits the cones, it stimulates them to varying degrees. The resulting signal is zapped along the optic nerve to the visual cortex of the brain, which processes the information and returns with a color.