Colour Contrast Analyser is described as 'allows you to evaluate the color visibility and contrast of foreground/background color combinations. It provides an initial pass/fail assessment against WCAG 2.0 color contrast success criteria'. There are four alternatives to Colour Contrast Analyser for Online / Web-based, Self-Hosted solutions, Electron / Atom Shell, Firefox and Mac. The best alternative is Who Can Use, which is both free and Open Source. Other great apps like Colour Contrast Analyser are Palette for Mac (Free), Wave (accessibility tool) (Free) and Cron Color (Free).
Palette provides a super simple way to analyze colors across RGB, #HEX and CSS formats. Try it online for free or download it for macOS with a donation-based download where you pay what you want (even $0 to try it).