Palette Ninja is described as 'Create harmonious color schemes' and is an app in the Photos & Graphics category. There are more than 25 alternatives to Palette Ninja for a variety of platforms, including Online / Web-based, Mac, Windows, iPhone and iPad. The best alternative is Just Color Picker, which is free. Other great apps like Palette Ninja are gpick (Free, Open Source), Paletton (Free), Colormind (Free) and ColorImpact (Paid).
Pika (pronounced pi·kuh, like picker) is an easy to use, open-source, native colour picker for macOS. Pika makes it simple to quickly find colours on screen, in the format you need, so you can get on with being a speedy, successful designer.
ColorMixer is a powerful application for mixing colors. You choose a color and ColorMixer will calculate 8 harmonic colors based on the one you’ve chosen. It’s user interface offers lots of utilities to choose colors.