
edit: what happened there? i was saying that i use the colour pick tool to get the details on patches that should be about the same colour, then play with the curves to get them aligned. but something odd happened. anyway, that's basically what i do.
sometimes it still bleaches so i shift the curve back slightly.
do this for a highlight, mid-tone and if necessary something in shadow and it matches fairly well.
it works best if there's actually grey on both surfaces; greys are always pretty much neutral, so you should have something like rgb=134,133,136 for a patch of grey. here i used skin that was almost bleached out and mid-toned.