change my background color in final cut x?

Background of what? The application? You can't.

You can change the clip background color in the Preferences > Playback > Player Background.

Are you talking about the Ad Color Mask and ad Shape Mask? If so it is on color the board - Correction

Do you mean to change the black background in the player window? You can change it in the preferences to either white or checherboard.

If you actually want to have your content superimposed onto some other color background, you should use the custom generator, connect it below your primary storyline and adjust its parameters however you want them.