Hi @Rob_Rose yes, you should be able to control the default text color for math expressions. If you’re using MathJax you can go to Settings → MathJax and changing the text color to your preferred hexadecimal value (https://YOURBOOKRUL/wp-admin/options-general.php?page=pressbooks_mathjax).
If you’re using WP QuickLaTeX, you can set the font color in ‘Basic Settings’. Same idea:
I’m not 100% sure which theme you’re using, but you ought to be able to use the browser inspect tool to find out the hex value of the blue color used in the caption and enter the same value so the math expression matches. See Manipulating Colors with Color Picker in Chrome DevTools for a quick guide if you’ve never used the ‘Inspect’ tools before.