There is an inconsistency with respect to colors available within ArcGIS Online. While the same color options are present when changing colors for polygon outlines & fills, only a reduced color palette is available when attempting to change the color of label masks. The full range of colors should be available across all symbology dialogs within AGOL.
I'd also like to lobby for the addition of user input of hexadecimal values. There already is an Idea submitted for this here.