Lasso Tool

Lasso Tool is a freehand selection tool. Draw an outline with lasso tool to select an area.
hotkey: L

Modifier Keys
Ctrl - hold to Add to an existing selection.
Alt - hold to Subtract from an existing selection.

Color Fill is an option of Lasso Tool, available in Tool Options menu; allows to automatically fill selected area with current color.
This works for both: layer and mask.
Layer - color fills to spree
More on Spree here
Mask - affected directly, color does not matter, since masks are alpha channel based
More on Masks here
Additionally using modifier keys subtract function, while Color Fill option is active - also allows to erase spree and mask contents.

Selection continued...
More information on Selection feature here