Color scheme settings interface is missing

When at the location . . .
Home » Administration » Appearance » Colorizer (for "Flat UI")
. . . a box labeled "Color Scheme Settings" is visible but it does not contain/show any controls for modifying color settings.

I have these modules installed:
colorizer
radix_colorizer

I have also installed Radix Stanley, another Radix-based theme, on the same site. When Radix Stanley is enabled, controls for modifying color settings are displayed in the Color Scheme Settings box.