ColorStrokes is the most popular and feature rich selective coloring photo editor on Mac App Store. It was released in 2011 and is ahead of any similar app since then.
Get tons of artistic tools for hassle-free color splashing, including possibility to apply custom colors, separately enhance background & foreground, add dramatic effects and more.
Selective Coloring Brush with adjustable parameters
Full Color Palette to Add new colors
Custom monochrome backgrounds
Selectively enhance background and foreground