first, the black rectangle as Color 0 or Color 1 (both are defined as black in your palette, not sure why you have the same color twice in there);
secondly the white G with the color index we just defined, in this case Color 3:
Then it works, here is a screenshot in InDesign with a cyan circle in the background:
PS: pssst, consider correcting your path directions: select all glyphs, hold down Opt (Alt) while you choose Paths > Correct Path Directions for all Masters.
Any color you want to use in a CPAL/COLR font (or an SVG color font derived from a CPAL/COLR setup) needs to be defined in a color palette. Take a look at the color font tutorials to find out more.