Hello! I am unsure if there is a way to change calendar colour settings in that way, unless you download a launcher or third-party theme pack.
Does it matter which calendar is currently checked to display in the Google Calendar app?
Just to be clear -- you're talking about 2 different calendar accounts in your Google Calendar? Or are you talking about 2 different calendar apps?
Yeah, the At A Glance widget will specifically link to the primary Google account's calendar