Thanks for trading!
Yep, I know what you're talking about. Did you add layers in? You know the 'lock Alpha' thing? I don't actually know how 'lock Alpha' works, but if you create layers, you can colour in the background. If layer2 (background) is above layer1 (mermaid), then the background will cover the mermaid but if you move it down, the mermaid will be on top of the background.
Layers are really useful when you're drawing new things on your art

(if this is unclear, I'll do a screenshot of how the layers work)