Color theory: https://en.wikipedia.org/wiki/Color_theory
Online Color tools
☞ Adobe color lets you test and save palettes.
☞ Color palette generator by Canva, picks a palette from any image that you upload yourself.
☞ Color Hunt Color Palettes for Designers and Artists Color Hunt is a free and open platform for color inspiration with thousands of trendy hand-picked color palettes
☞ Material Design Color Tool Create, share, and apply color palettes to your UI, as well as measure the accessibility level of any color combination.
☞ Paletton Paletton is an online application formerly called Color Schemes
☞ COFFEE is the color, find out what real words are valid CSS HEX colors.
☞ http://bada55.io, similar to COFFEE: find leet words for your CSS hex colors.
☞ Open color Open color is an open-source color scheme, optimized for UI like font, background, border, etc.