My favourite site for colour-blind testing has been taken down. I’ll find a new one when needed.
In general, bright vs dark is safe. Always use something in addition to colour to make things stand out, such as size, font, underlining, italic, framing, or spacing.
This tool, written by my husband, lets you see how colours look together. Often a colour looks different depending on how much there is of it, and what’s around it. Enter colours using hex or HSL. To copy and email the page with your colours, right-click on Link, then Copy Link Address.
This site tells you the colours of your favourite picture.
This site lets you play with different colour models and see how different colours look together. The beta version also shows you colours that are mathematically related to your chosen colour.