Question

why a red rose appears black in the green light and red in the white light​

Answer 1

Objects appear a certain colour because only certain colour's of light are reflected of by them. Similarly a red rose reflect red light of its surface making it red. When a red rose is seen in green light, it absorbs the green light and the petals appear black as no light is reflected by the rose.

Answer 2

Red rose in green light appears to be black. This is because the rose absorbs the green light falling on it and reflects none.  This is because the rose reflects the red light falling on it and absorbs none of it.