. . . . . . . . colour of light favors stem elongation which unfavorably resemble etiolated plants.

The red colour of light favors stems elongation which unfavorably resembles etiolated plants.
Etiolation is a process in flowering plants grown in the partial or complete absence of light. Lack of sufficient light retards chlorophyll formation and promotes slender growth with long internodes, thus leading to pale green leaves, spindly growth, and premature drop of leaves and flowers.
This process is regulated by the exposure of various photoreceptor pigments to light. Phytochrome A and phytochrome B both respond to an increasing proportion of red light to far-red light which occurs when the shoot comes out into the open.