Use the Color Brush to add brush strokes to the Colorize layer.
Use the Eraser to erase brush strokes from the Colorize layer.

Use the color picker to select a color to paint into the image. The hue and saturation of the color can be adjusted through the color picker.
Note: Only the color's hue and saturation will be blended and painted to the image. Using any black or white shades will simply turn the colored image into monochrome, and will not have any effect on black and white images since they do not have saturation at all.

Use the Size slider to adjust the size of the brush.

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.

Use the Strength slider to adjust the opacity of your brush strokes. Lower values are more transparent, and higher values are more opaque.