Cannot change footer color to #000 (black)

In this app, either the ArcGIS Online hosted version or the downloaded version, it is not possible to choose the custom color scheme and change the footer color to #000 (black).
*#*000001 will work, but not typing or picking #000 from the color picker.

Update: Apperently this also affect red (#f00), and white (#fff) - I haven't tested it myself
BUG-000093041