PamH, The colors you have are good, or use white, as Judy suggests. It would diminish the overall look if you add another yellow, IMHO. The yellow is quite strong, so it depends on whether you want a sedate quilt or one that "pops!"
Wish I'd asked for folks to tell me if I had something in the wrong place...I keep looking at it & thinking there's a mistake in the way I sewed the star together. Too late...no way am I ripping it out!
Good sewing & be sure to show us the final result!