You can easily change the color of the by clipping colored layer to the calendar.
First, create a layer above the calendar.
Then choose the Paint Bucket Tool.
Fill the new empty layer with the color that you want.
Clip the color layer to the calendar above by going to Layer –> Create Clipping Mask or the shortcut Alt+Ctrl+G.
And tada, the color of the calendar is changed.
Now you can adjust the color of the calendar according to the color scheme of your page.