Most leaves have the upper surface more green and shiny than the lower one. Why?

The concentration of chloroplasts (implying more chlorophyll) and stomata are more on the upper layers of the leaf which help to trap the sunlight really fast. The layer of the cuticle is thicker in the upper epidermis. Thus, the upper surface of the leaf appears greener than the lower.

Explanation: Higher concentration of chloroplast concentration the upper layer of leaf signifies a higher concentration of chlorophyll in those layers. This chlorophyll helps in absorbing light energy which is required for photosynthesis. The chlorophyll absorbs all wavelengths of light except green, which it emits. Thus, the upper surface appears greener.