How do I make an in game paint tool with brush size and the color of blocks be able to be copied?

Ingame paint tool to color blocks, be able to increase brush size and be able to copy the color of the blocks that it's hovered over with a key as an example: [E,C,P]. Also it needs to be visible to other players. [Take the paint tool from World Conquest or Risk Universalis 3 as examples.] I first added a gui and did all the stuff needed to make it in to a tool, being equipable so that you can basically equip the gui right away and start painting with an image button, but still I can't find what to do after that as I have been stuck there.