Dogs have two types of color receptors (cones) versus our three, so they don't perceive the full rainbow. Their vision is roughly equivalent to red-green color blindness in people.
What this means in practice: dogs see blues and yellows well, but reds and greens blend into grays and browns. A red ball on green grass is nearly invisible until it moves.
Dogs compensate with far superior motion detection and night vision, thanks to more rod cells and a reflective tapetum lucidum behind the retina.
Practical tip: choose blue or yellow toys for visibility, and don't assume your dog "ignores" a toy — it may simply not see its color against the background.
