Currently, the fill tool works with any color besides
black. This is because the "default" color for a
BufferedImage pixel is black with full transparency
 (0, 0, 0, 255). If you want to try out the fill tool,
change the DRAW tool to paint with color, then fill
inside of that. Filling WITHIN black borders does not
work. Filling with the color black does not work. Work
with colors, and it works.