I have seen this used quite a few times in novels. Two examples are:
Molasses buckets appeared from nowhere, and the CEILING DANCED WITH METALLIC LIGHT.
-To Kill a Mockingbird
Aureliano...sitting against the metallic and quivering light from the window...
-One Hundred Years of Solitude
I am thinking that it means the light glows as if reflected against a metallic surface. Is this it? Are there any underlying meanings?