I've used the crock pot method for quite a number of Magnum and OS engines with excellent results and no change of color. I always thought the color change was associated with getting the antifreeze too hot. I doesn't have to be boiling or anywhere close to it actually. Keep the setting on low, if it has one. Mine have all come out new looking with no brushing or anything except rinsing them off and oiling them up after an overnight session in the crock pot with undiluted antifreeze.