If you cut yourself (or spear a fish) 30 ft underwater, the cut bleeds a bright emerald green. But as you draw closer to the surface, the blood turns brown, then pink, and finally red at the surface. Why does it change colour?
In fact, the blood doesn’t change colour at all. What changes is the light in the water.
White light, like the light from our sun, contains all the colours in the spectrum: red, orange, yellow, green, blue, indigo, and violet. A white piece of paper looks white because all the colours are bouncing off it and reflecting into our eyes. A red thing looks red because red light bounces off it, and all the other colours are absorbed. A black object is black because all the colours are absorbed and no light reflects into our eyes.