Painting, fills and gradients

Painting tools work on pixel layers and on masks. Add an empty pixel layer with Layer → New Layer before painting on a new document.

• Brush (B) — size, hardness and opacity. [ and ] change the size.
• Pencil (N) — hard-edged pixels, good for pixel art.
• Color Replacement (Shift+B) — paints the foreground colour's hue over colours like the one under the first dab, keeping their shading.
• Eraser (E) — erases to transparency.
• Background Eraser — erases only colours close to the one under the first dab (Tolerance), for cutting out an object from a plain background.
• Magic Eraser — click to erase an area of similar colour at once, like the paint bucket in reverse.
• Paint Bucket (Shift+G) — fills an area of similar colour with the foreground colour.
• Gradient (G) — linear or radial, from the foreground colour to the background colour or to transparent. Shift snaps the direction to 45°.
• Eyedropper (I) — click to pick the foreground colour, Alt-click for the background colour. Alt-click with the brush or pencil also picks a colour.
• Color Sampler — up to four fixed points whose colours stay listed in the options bar while you edit. Drag a point to move it, Alt-click to remove it.
• Ruler — drag to measure a distance and an angle (Shift: 45° steps).

X swaps the foreground and background colours, D resets them to black and white. With a selection, all of these tools stay inside it.