Question

I have known that plants need only blue and read color for photosynthesis. Why those colors are useful for them to produce food ?

Answer 1

Here's my best guess:

As you know, plants need a certain amount of energy from sunlight in order to carry out their light-dependent reactions. To trap this energy, molecules such as chlorophyll trap blue and red wavelengths (and reflect green ones). I don't know exactly why red and blue light were used particularly, but I can point out some minor hints to hopefully help you a bit.

The answer *might* lie in the habitat of the plant. The interesting thing is that photosynthetic organisms are not universally the same, appearance wise - in the plants we are familiar with, green is indeed the dominant color. However, in red algae, red is - they have pigments known as phycobilins which help them absorb a variety of colors (including green). This feature represents, I believe, an adaptation to their lifestyles; they generally inhabit deeper regions of the ocean, and thus require more light to conduct photosynthesis.

So if we take this as a clue that the colors used by a plant for photosynthesis is linked to its habitat, then there may be an underlying factor selecting for the usage of specifically red and blue light. (maybe these wavelengths are optimal; purely speculating here :P)

Alternatively, perhaps chlorophyll simply evolved this structure, and because it worked, it was readily incorporated into the photosystems of plants. Other pigments were selected against, presumably because they absorbed too much light (see: photoinhibition) or too little, and thus were outcompeted.