For the Tags field (not a custom field), the colors of the tags are auto-assigned by Aha!, without opportunity to control the colors (as far as I can tell). Many of these tags do not meet basic color contrast ratios to meet accessibility needs.
A contrast ratio of 7:1 is needed to meet WCAG 2.1 AAA conformance.
Thank you for your idea. We have shared your feedback with the team. In the meantime, account administrators can change colors for standard tags by navigating to Settings ⚙️ -> Customizations -> Tags. If you run into any questions while trying this out, please reach out to our Customer Success team at firstname.lastname@example.org.