Black will let the color stand out the best. Colors other than true gray (equal quantities of r,g,b) will more than likely clash with the colors of the art. Black, white and gray, are great because it is totally neutral so it goes with everything. (edit: Although I do not think that white will work to make a cool arcade background) As to removing the yellow, this is an easy one. If you go to Adobe's website, most of the time they have a 30 trial version of Photoshop. Even Photoshop elements will work for this. Grab the magic wand tool and select the yellow. Then in the menu, select fill with and select black. You might have to go around the border and make a couple of little touch ups as the magic want sometimes does not get everything. Personally I would go at it with paths and make a vector mask, but if you are just learning, magic wand is your friend. If you continue to learn PS, be careful not to let the wand become a crutch.