Why are dark irises of eyes, such as brown, harder to dilate than light-colored irises such as blue?

Dark coloured irises contain more pigment cells than the paler (blue) irises. The Pigment cells absorb the drops administered to dilate the pupil, and obviously if there are more cells, then more of the chemical is absorbed. Therefore paler eyes dilate quicker.
Popular drops used are Tropicamide 1% and Phenylephrine 2.5% although 10% can be administered for very resistant eyes.
Reference: Professional expertise and discussion with Ophthalmologist.