I soaked the belt in the oxyclean solution and then pulled them out and scrubbed them with a soft brush and rinsed.
For standard belts, remove the plastic buckle covers *before* soaking. The oxyclean removed the color from the top of the covers and from several of the release buttons. The sun had faded the covers and cleaning apparently pulled out the oils/plasticizers that were keeping the color darker. The undersides were fine. I sanded the center section and removed the light colored plastic.
Here's a pic of an original and a cleaned one (both are blue). Not sure what to do about the textured sides.
fwiw - To remove the cover, insert 2 small screwdrivers down the sides closer to the buckle's opening and pry up.