How would I go about creating a gun that can spray liquids on walls that permanently stays on the walls?

I am moderately new to game developing, I have made mostly 2d games that are pretty simple for the most part. But I think I am ready to move a bit beyond that and I am working now on a smaller survival game, where the character is in a dark warehouse, and in order to see the world around, you must put glowstick liquid into a water gun and spray the environment, illuminating the walls and the floor you shoot, permanently illuminating them for the duration of survival. I may change that in the future, so that it gets dimmer over time, but right now, I am just trying to figure out how to make a gun that functions this way. Can anyone help me out with this code? I am working in C# in unity 5.6.1. To anyone who replies, thank you SO MUCH.