You could use a custom palette though, I always recur to them when I encounter a problem of that sort. This might have taken you quite some time, so let me say this before anyone else, good work, your work is finally done. (Y)
This background could have many uses, even without themed recolours. It feels true to SMB3, and although the original NSMB BG is yellow, I think it looks better in an orange tone actually. The hills just seem to pop out more with a richer, darker colour.