The rods and cones in your eyes are sensitive to the colours red, green, and blue specifically. It’s why LEDs, televisions and monitors, etc use those colours. All other colours are some kind of blend of these 3 colours. For example, yellow is red+green in about equal amounts.
Objects are considered to be a colour based on which of those 3 main colours they do or don’t reflect. A red apple will only reflect red light. Under a green or blue light, the apple would look almost black. “Almost” because these colours and the object’s reflections of light are rarely perfect.
But something in between will behave interestingly. A yellow object reflects both red and green colours. So if you shine a red light on something yellow, it’ll look red. Same for green. Both together and you’ll see the actual yellow.
Lacking more detail, I think this is the effect that you are seeing.
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