The brand color accessibility contrast isn't high enough today; how do I talk to the client about changing the color?

I delivered a landing page version to a client today. The main button used their brand orange with white text, which looked good visually, but the WCAG contrast wasn't sufficient. The client didn't want to change it at first, saying the brand manual specifies it that way. Instead of just saying 'it's non-compliant,' I showed them real mobile screenshots and the contrast ratio values, then…