Light absorption in photosynthesis involves substances that are called pigments. The main pigment that is used in photosynthesis is chlorophyll
Pigments that absorb all the colors appear black, because they emit no light. Since chlorophyll appears green it would be obvious to see that it absorbs red and blue and reflects the color green which is why plants appear green to the human eye.
Hydrogen is needed in a later stage of photosynthesis to make carbohydrates. Splitting water also releases oxygen, which is a toxic waste product and is allowed to diffuse out into the surrounding water or air.