A published Effect will always be a composite once you’re in FCP X so you can’t strictly do what you’re asking.
However you could make the your Motion project a Generator instead of an Effect which will mean your background won’t be included and hence you will be able to color correct it to your heart’s content (or add any other filters to your background layer).
That’s assuming that you don’t need your Motion effect to influence the background clip in any way.
If you do want to affect the background clip as part of the overall effect, why not publish some color filters from Motion for that layer (e.g. Color Balance, Gamma, Saturation, etc.) and then you will be able to control the color inside FCP X.
You could also consider making this a Transition as this sounds more suitable to what you’re describing – but without more details it’s hard to be sure.
Simon Ubsdell
http://www.tokyo-uk.com