In FCE 4.0.1, in the Browser window, select the Effects tab > Video Generators > Matte > Color. Double click 'Color'.
In the Viewer window, select the Controls tab, click the swatch (the square color box), and the color wheel comes up. There are also sliders for the hue, saturation, and black level if you're old school.
Then, in the Viewer window, select the Video tab. Click anywhere on the color you chose, and drag to the time line.
If you put the color on track V1, place your Text track above it in V2. The black from the text track will be replaced by the color track you chose. The text and colored background being stacked as 2 tracks are called "nested" / a "nest" / "nested tracks".
Good luck!