Thank you for bringing this to our attention! I apologize you're having this experience with updating the color on your form. I can understand how important it is for you to easily create a sign-up form through your account. With that said, you mentioned applying a custom hex code doesn't work. What exactly doesn't work (error message, apply button unresponsive, etc.)? Having any further details on what you're experiencing is important information for us to have so we may troubleshoot further for you.
Thank you for these clarifying details! We were able to determine this is a defect you are experiencing with our landing page sign-up forms. This is something we are continuing to troubleshoot. We have also added you to our queue to be notified when it is resolved. A workaround we suggest in the meantime is to pick the closest matching color you can. I truly apologize for any inconvenience this may cause.