WebAIM: Contrast Checker

One of the easiest contrast accessibility checker tools out there. This is a great option if you can't or don't want to install something on your computer. My favourite thing about this one is how pedagogical it is. If you spot a problematic use of colour and then stick the hex values into this thing and take a screenshot of it, that screenshot contains everything you need to make your point to whatever colleagues and stakeholders need convincing.