How can I pick a color in Ubuntu 18.04/Unity 2020.1.3fl?

I'm experimenting with Unity 2D as a possible platform to build a card game app. I'm an experienced programmer (Python/Javascript).

I started with

and am happy to dig into C# (rider installed) as necessary.

I've hit an early blocker with the color picker in Unity which does not seem to work at all. The tutorial creates images to differentiate areas of the table top and initially uses the color to differentiate them. If I click on the Color property or dropper, no picker appears - the white just reduces to the lower third of the property.

Does anyone know how I can pick colors for images in Unity 2D in Ubuntu?