For an artist, go to the artists section and edit the artist in question. Find the “colour” option, and click on the coloured box. This should make a colour selector appear that you can use to choose the colour for the artist’s calendar. Save the artist once you’re finished. For other custom calendars you’ve added it is much the same, except you simply edit the calendar in question from the Calendars & Subscriptions menu of your Marcato calendar.