Use this tool to colorize the original image using a color of your choosing. White and black areas will remain unaffected, grayscale areas will be colorized. The "Hue, Saturation - Colorize" filter with activated Color option does the same.
If this option is active, all layers will, internally, be combined to on. The Colorize tool will take into account all layers but will only be applied to the active layer.
Otherwise the Sharpen tool works like any other normal brush. The Colorize tool’s Attribute Manager contains all brush parameters, as already described here.