calendar colour selection in Mojave

I am using Catalina with Calendar Version 11.0 (2760.4.1), so the following Screenshots may not exactly match your version, but the first screen shot displays the "colored circles," and the second screen shot displays the "Custom Color..." palette:



You will have to right-click on the calendar name to see the "Get Info" window, or use the (⌘+I) keyboard shortcut to display this window:

According to: Change a calendar’s name or color on Mac - Apple Support

"Click a colored circle to select a preset color, or click Custom Color to select a color from the Colors window."

Depends on what calendar you want to change. Also in that article:

You are welcome.