Why a yellow lemon may appear green
Strange things happen when the light shining on an object is not white light. If a cyan (greenish-blue) light is shone on a lemon, the lemon will appear green.
The lemon’s surface normally reflects red and green. There is no red in the incident light, only green and blue, so the lemon will reflect green. The surface will absorb blue, just as when the white light shines on it. So overall, the lemon under cyan light is absorbing blue and reflecting green.
This diagram makes the process easier to see: Think about the composition of the light that is shining on the object, and then consider which colors are absorbed.