This is my go to tool! And actually my main tool used when drawing!
This is the brush I use to select my base colors!
How to use:
Step 1: Select outside of lineart or CLEANED/RENDERED sketch
Step 2: Invert selection and use BUCKET button to fill in!
That's it! If your wondering why the sketch has to be renderd for it to work best is because it selects ALL so it's best to have you Sketch/Lineart clean!