Answer:
Plants use the red and blue light from the visible light spectrum; these are captured in the wavelengths between 440 and 660 nanometres. Green light is reflected, giving plants their green colour. Day length also plays an important factor in the effectiveness of photosynthesis.
Since red is the most abundant colour in visible light, then it could be referred to as the "most useful" since it is the one used most.
Bear in mind that photosynthesis is not the only process involving plants and light, since insect pollinated flowering plants (angiosperms) use bright colours to attract insects and other animals, in which case, light is required in many forms, not only visible, but some ultraviolet, since several insects see by means of ultraviolet light.