Grease Pencil: color picker locked up? / likely noobie errors? / blend attached

I’m working thru a GP tutorial ( ) and trying to alter the color on a fill operation. Sadly, the color picker isn’t responding at all, nor is it filling in the color that is displayed in the color swatch (it’s using black).

What are the likely user errors here? Noob errors. Like, I already checked to make sure the Material had the 'Fill" box checked.

Here’s the blend in question:
GP_tute_2.blend (4.2 MB)