Click the flag to start! Click colors to draw! Click rainbow mode to activate it! TIP: If you want to get rainbow with light theme, select eraser first. if you want to draw with normal colors after using rainbow mode, you have to click quit rainbow mode. Click clear to clear drawing. Click eraser to erase. To change size, hold down number button while drawing. However, 0 sets the size to the sizes available in the menu. M to open Menu.
All thanks to @HannahPengiun315. Just fixing some bugs.