[Nekobai]

You can test individual colours using tools like https://webaim.org/resources/contrastchecker/

To check the colours on a page (and many other accessibility violations) you can use a bookmarklet like HTML Codesniffer ( https://squizlabs.github.io/HTML_CodeSniffer/ ), the tools that are integrated with Firefox and Chrome's devtools, or CLI tools like Axe or Pa11y.