More data and information is encoded in color that can be transferred more rapidly telling of danger, mating, shelter, food faster and more selections than just black and white and greys.

The Why is because it is favorable in every category except the requirements of more energy to build and reproduce a much more complex eye. But the chance to find the food requirements is fulfilled rapidly.
Its a very good cost per quick payoff reward mutation, as well as long term.
Forward facing predator eyes.
Eye lids
Focus.
Left/right, up/down range of gimbal tilt motion in the eye sockets, that is set in a head with up/down, left/right gimbal tilt motion, set on a body with left/right, forward backwards, up/down gimbal movement.