Hey Amanda,
I took a quick look and did my best to solve the problem, but we’re limited on what we’re able to support and this unfortunately extends beyond our Forum Guidelines. When viewing the ‘Month’ view, the opacity for dates that have passed and the next month’s dates are turned down to 0.5, so that might have something to do with it.
It looks like you’re using The Events Calendar Category Colors, so I’d recommend contacting the authors to see if they have any insight on the matter. If you still can’t get it resolved, we have a list of customizers that might be able to help.
Thank you for understanding and best of luck. Have a great weekend!
Cheers!