I noticed when I edit the color of a feature on Map Viewer (not the "classic") only the Color Ramp (see attached) is available. I do not see to option of "traditional" Color palette (the one that usually occurred on ArcMap desktop, ArcGIS Pro & "map viewer classic").
It seems that other than Color Ramp, Color Picker is the default--& only option for Map Viewer available. While it is (theoretically) more powerful, in practice it's not that user-friendly--who is going to remember the combination of RGB or HSV of every color they use? Moving the cursor on Color Picker in order to "pick" a good color is too time consuming & impractical.
With the "old fashioned" Color Palette, one can just pick & choose some "standard" color for their mapping purposes. Color ramp on unique value, on the other hand, has too few options.
If not, I just hope ESRI can add the "classic" Color Palette to Map Viewer as an option--who knows when "classic" will retire like the way ArcMap does? (Yes ArcMap is FAR SUPERIOR than ArcGIS Pro--NO APOLOGY)
Ken