It may have a brightening effect, visually, because it’s only removing the organic material embedded in the stucco, but it shouldn’t be washing out/lightening the color if thats what you mean? No, to my knowledge SH is not he chemical of choice for oxidation… there are various low and high cost oxidation removal chemicals that work to varying degrees with and without agitation (la awesome, ox nox, cleansol bc, etc). Give a couple of these threads a look and see if they help.
