Hi,
The "Colour" and "Gradient" tabs of the pop-up generated by clicking the colour swatch in the Fill and Stroke sections of the Appearance panel are semi-transparent, allowing the underlying document to show through, while the "Swatches" tab is fully opaque. This is inconsistent with similar tabs generated by clicking the contextual toolbar or the Colour panel in Studio: all of these are fully opaque.
The video shows the effect in Light mode, but it's similar in Dark and Default OS modes too. I'm running MacOS Mojave 10.14.3.
appearance panel semi-transparent.mov
Cheers!
ADD: this is linked to the "Reduce Transparency" setting in System Prefs - Accessibility - Display. When this setting is checked, the background of the AD panel is fully opaque.
It's still inconsistent with the other panels, though I suppose you could argue that in fact it's the other panels that aren't respecting this setting in System Preferences.