Chanigng a material or decal with spot light


I was wondering if any of you can help me figure this out! I have made a flashlight that my player can pick up, it has two functions; normal mode and an ultraviolet light mode (just a spot light with a purple colour). I was wondering if it is possible to show hidden messages when the UV light is on via decals or changing the wall material when the spotlight is shining on that area?