well you have to remove the background first with this snazzy magic wand tool:
(this tool selects areas of similar colors. so if you were to click on something red, it’d only make a selection on the red part and not the yellow part next to it.)
select all the areas you want to erase. (i recommend clicking all the unwanted areas while holding the Shift key bc without doing so it takes wayyy longer)
when you’re done selecting, (DON’T deselect anything yet) click on the erase tool and it’ll only erase parts within your selection. pretty simple stuff! you might have little unwanted parts after you’ve erased but you can clean those up later.
and then make a new layer below the picture and fill that layer with whatever color you want. OR just add a solid color and put it below the picture.
*rewatches the scene that sifu-korras pointed out* (x)
not trying to rain on your parade but it could be the fact that lin and suyin were gonna BLOW UP if bolin didn’t land the shot in time