Name the pigment, which is responsible for red color light?

Photomorphogenesis is a developmental process in plants that uses light to control growth, development, and differentiation. It's a separate process from photosynthesis, which uses light as an energy source. 

Photomorphogenesis occurs after seed germination when seedlings undergo de-etiolation after exposure to light. Cryptochromes are involved in many aspects of blue light-stimulated photomorphogenesis. 
Photomorphogenetic responses are also important to older plants. Many such responses respond to the relative lengths of day and night.