I’m leaning more towards Ed Slushie’s point and would go pale blue, unless it looks super weird.
I remembered there was a cool red light effect on Yoda and Ben Kenobi in ESB where Luke takes off from Dagobah and thought that might give us an in-canon example. But Ben vanishes before the red light covers that area, even though he’s still talking to Yoda, so looks like Lucas didn’t know what to do either 😃 Ben was shown at full brightness when his surroundings were dark, tho, so having the Force ghosts be somewhat independently lit/colored from their surroundings feels like a reasonable avenue to go in.