Photosynthesis

CO2 + H20 -> C6H12O6 + O2

Thats the equation for photosynthesis. 

Chlorophyll helps photosynthesis happen; and chlorophyll has many pigments: green (majorly), red, blue, yellow, orange...

To make photosynthesis start, you need light (red, orange, yellow, green, blue, indigo, violet = white)

Green plants use most white light except green (that's why the plant is green).

-If a green plant is put in green light, then it will not do much photosynthesis (hypothesis).

In the ocean, light penetrates, right? So red, orange, yellow and purple goes out the first (which is what normal green plants use).  Blue goes the deepest (that's why the ocean is blue) (Blue penetrates and reflects the deepest). Therefor, because there isn't red or yellow or orange light, plants need to be a different color (not-green) to absorb the light. And thats why deeper algae is brown.