Thanks! :)

• Make sure that the top of the GIF you’re going to make is a block color (any color you want).
• Follow this tutorial, but erase the whole(!) duplicated layer (at the last step).
• Click Image (at the top) > Canvas Size…
• This will appear. Click the arrow down. Click Ok.
• If you did all of that, this will happen. Select the brush tool and brush all over the part you want that color.

Done, I think. If you still have any questions, feel free to ask me.