The Colorize Tool lets you paint colors into an image using a brush tool. This can be used to add color to black and white photos.
Brush Settings

Brush Type
Use the Color Brush to add brush strokes to the colorize layer.
Use the Eraser to erase brush strokes from the colorize layer.
Brush Color
Use the embedded color picker to select a color to paint into the image.
Note: The colorize layer will only change the hue and saturation of colors in the image. Using a completely desaturated color may yield unexpected results.
Brush Size
Use the Size: slider to adjust the size of the brush.
Brush Hardness
Use the Hardness: slider to give your brush strokes hard or soft edges. Higher values will give sharper edges, and lower values will give softer edges.
Brush Strength
Use the Strength: slider to adjust the opacity of your brush strokes. Lower values are more transparent, and higher values are more opaque.