[whpalmer4]

Open a new document. Open the canvas sidebar, if not already open. Click on Canvas 1. Bring up Style: Fill inspector via Inspectors menu, or cmd-1 plus click on Fill tab if needed. Make sure checkbox for Fill is ticked. Selector beneath Fill checkbox to Solid, if not already there. Double-click uppermost color box to left of vertical slider and choose your color.

I didn't try this in 5.2.1, only in 5.2.3, so I suppose it might be somehow different there. Any reason you aren't using the current version?