I recently went to a photoshoot where the photographer asked me to pick a color for the backdrop and showed me 4 different colored filter looking things...(lol). The backdrop was gray... I didn't pay attention to how he put it onto his camera, or if he even inserted it at all (I was thinking about other things). I was woondering... What were thoes filters? What are they called? How does the photographer apply it to the camera/photos? And how did he add my color of choice onto the gray backdrop without tinting the entire image? Is the original gray backdrop a special backdrop? I'm very new and clueless to all this. Thank you for your time, help, and patience.