When defining a colour override for any colour property of a keyboard or key or keygroup, you can use a hexadecimal RGB string such as BackgroundColor="#709070" (i.e. "RRGGBB") or a named colour such as `BackgroundColor="AliceBlue". A list of suitable named colours is found at Windows brush colour names.

Note the US spelling on "Color" and on colour names such as "Gray".

For picking your own custom colours, you may find it helpful to use a Colour Picker to find the correct hex code.