can a red or green colour cloth be used to cover a portion of a leaf instead of a black one inorder to prove that light is necessary for photosynthesis ?
Black color absorbs all light falling on it. It does not reflect any or allow any light to pass through it. The idea of covering leaves in a photosyntheis experiment is to not allow any light to fall on the leaf and study its effect. If we use green or red colored cloth, they will absorb some and reflect some wavelengths of light. Thus an absolute no light condition will not be created and hence the results will not be as expected.