You have three (3) options. Let's walk through each of them.

Step 1: After you log in, click on Calendars (brown box below).

The Your Calendars page will open.

Step 2: Choose the calendar you wish to modify (brown box below).

Step 3: Click on Design (brown box below).

Step 4: Scroll down and click on Navigation (brown box below).

Step 5: Scroll down until you find the header Selected date.

If you don't choose a color, the month and year will be grey.

Click on Inherit from website (brown box below). In this option, the color is inherited from your website or other embeddable component. In this case, the month and year are now black.

Click on Custom (brown box below) and then click on the circle (brown arrow below) to choose your color. Note that now the month and year are blue.

Here is a walkthrough of this option.

Click Save at the bottom of the column to save your changes.