Karen has blue eyes, whereas tom's eyes are brown. their different eye colors can best be attributed to

This is due to the differences in pigmentation of the eyes. Melanin is the main pigment in humans and it's presence in the stroma of the iris result to brown to black iris. Low pigmentation causes lighter color of the eye. The appearance of lighter color is due to phenomenon referred to as Tyndall scattering of light by the stroma of the eye.