Ancient Relic: Photoshop 6.0

Hello. Ok, I'm desperate here...I still have only 6.0 and I'm trying to do color replacement. I've googled, You-tubed, used a book (remember those?!), but I can't for the life of me get a color replaced. Scenario: I'm putting friend's faces on Shrek and Fiona's bodies and I need to change their human skin tones to match the Ogres'. Please help!!! My book tells you how to do it, but it ain't workin'! Tell me it's not impossible.

Thanks in advance,

Todd