I'm trying to create a set of 6 text colours that are to be used on both white and dark purple (42185D) backgrounds (on a web page), and still meet accessibility standards (AA). Are there any tools or guidance that could help? Not a professional graphic designer so any help appreciated.
The W3AC provides standards for web accessibility of text. They require specific contrast ratios so it can be read by color blind individuals.
If you know the #hex for your background colors and text colors for your website, it's as easy as inputting them to see if they are AA standards compliant. They can be accessed via Snook Color Contrast Check
Not entirely trivial; but there are tools out there. You will find a list here: Is there a good resource or tool to help build a palette/color scheme around colors I select?
Edit: actually Color scheme designer might be a better fit for your use. It has a "colorblind" option.
One is Adobe Kuler, and you can play around with it like this: