While plants don’t use each color equally, they do prefer broad-spectrum light. From the lowest-wavelength (highest-energy) to the highest-wavelength (lowest-energy) these colors are:
Ultraviolet (UV) light causes many plants to develop thicker cell walls, pigments, vitamins and other substances in a process called photomorphogenesis. In plants that naturally develop red pigmentation in their leaves such as red-leaf lettuce, UV light is responsible for the pigment production; without it the plants will remain green. In Cannabis, UV light is responsible for increased THC, CBD and terpene production.
Blue light is used directly in photosynthesis, and is partially responsible in triggering plants to grow compactly rather than “stretching”.
Green light is mostly reflected by leaves, which is why plants look green to us. However, some green light is used by plants for hormonal signaling and without it plants don’t grow normally.
Yellow light is absorbed by plant leaves but not directly used; yellow photons “bounce” inside leaves and lose energy in the form of heat until they become red photons the plant can absorb. Yellow light is helpful in heading leaves up to their ideal metabolic temperature, unless you compensate by raising the growing area temperature.
Orange light is used for some photosynthesis, photomorphogenesis and hormonal signaling.
Red light is the most readily-absorbed by plants and the most efficient for photosynthesis. However, without far-red / near infrared and blue light, plants given mostly red light will grow long, weak stems.
Far red / near infrared (IR) light is utilized by plants for hormonal signaling controlling flowering, stem elongation, and as a photosynthetic enhancer through a phenomenon known as the Emerson Effect.
Only with a balance of light across the spectrum from UV to IR will plants achieve their full potential. Green and yellow light are required, but in amounts significantly less than the photosynthetically-ideal red and blue light.