An easy way to do this will definitely make it to the app at some point but right now, although possible, it involves a few steps. Here's how I do it:
Open a Draw Layer on your canvas and zoom out until you can see it in its entirety
Tap the Shapes icon (circle/triangle) in the upper nav
Choose the square
Resize the square (two finger drag it until it fills your canvas
When it's the right size, double tap it to get the square outline (the shape will outline with whatever color is on the selected brush) then tap the X to close it
Long-press (press and hold) inside the square to fill it with color
Make sure that Draw Layer stays as your bottom-most layer
Hope that helps. Let me know if you have trouble.
Thank you, that will work for now.