Thank you for reaching out and sorry to hear you're having trouble using the color picker in XD. I am unable to reproduce the issue at my end on my windows machine. Please share your operating system information with me so I can test it on my end. Does that occur with all files or just one in particular?
Please try restarting the computer and signing out of XD, the Creative Cloud Desktop application, and let us know if it solves the problem.
I'll do everything I can to look into the situation.