I think a gold tone would look good, especially if you pebbled or did some other type small fill-in quilting stitch. That would make the tree stand out. Food for thought....I know you said you were going to do the tree in green, but why not silver like your star? That could make it almost magical. If you did decide to do the silver, then do the background in one of the colors in your background, maybe the light pale yellow. Just something to enhance the tree.