How do I remove a background from a Canvas Drawing?

Hi. I'm creating a game where players can colour in their own flowers and such. However, when uploading and posting the final result, the flower colours extend past the lines. Is there a way of keeping the colour within the lineart? gyazo.com/3060a90fd043be3ce6e3825a560c2786

I was considering some kind of clipping mask, but I don't know if that's possible in Construct.

Yes, check out the blend modes example. editor.construct.net

However, you will possibly need to modify your source file to have transparent areas.