I picked my own color palette for the redesign of cassey.dev, and this site called Contrast-Ratio.com was helpful in resolving a pesky contrast issue between the background color I was originally using and the link text.
Since I used HSL color encoding, I could adjust each component value of each color up or down, and get a related color to what I started with, as I tried to hit that sweet 4.5:1 contrast ratio for better accessibility.
(I should add: I was reminded why HSL colors are so great, and lots more, by reading Refactoring UI while I was working on the re-design.)
# update: one day later!
I came across this post on designing an accessible color palette with magic numbers by Darek Kay, which outlines a really great approach to defining a new palette that has consistent contrast ratios that can be easily understood by designers and developers. If you're starting from scratch on a palette, check it out!