Honestly, it's a notion he almost outright dismisses but then he has to stop and consider, head tilting some as he scowls and thinks. Isn't that just what the stained glass in church windows do? Colored glass blocks the light, technically, even if it does filter through.

Would the colors that light casts be considered a shadow? Only he can't ask using that example, now can he? A dog, new to sapience and art, wouldn't really know about stained glass. He does acknowledge the idea, though, so Joker gets that much.

"I hadn't thought of it but maybe that would work," he replies, "If it's thin enough or the light itself was a different color or something like that."