The Paint Tool is one of 6 main tools within the game. The Paint tool allows the player to paint and color most parts, adding pops of color to an otherwise boring craft. The hotkey for this tool is 5.
All materials can be painted by the Paint Tool, but not all parts can be painted (or all parts of them, for that matter.)
Every material in the game, painted a stark white for simplicity. Regular glass, while showing other colors, doesn't show white very well- hence its ordinary appearance in this screenshot.
The Paint Tool's appearance in the tool menu.