Start a game by clicking the "Single Player" Or "Play Button", Select your map, and make sure your game-mode is in Sandbox.

Then Click the green "Play" Button in the bottom right.

When loading in, you will see a progress bar at the bottom right.

This is supposed to be there, just let it go and don't click "cancel"

Once you see "Starting Lua" then you will be in the game very soon.

Once you are in press and hold your "C" key. Then click the Customization Option.

Go to "Physgun Color" And customize from there. (Hint: You can put the 3 color numbers into negatives)

Set all to (0,0,0) For a Black Physgun and an invisible beam. Etc.