Sorry if this has been asked before, my search-fu is weak.

I'm fiddling with the creation kit, just trying to get the hang of it. I decided to play with Taarie, fiddling with her appearance, inventory, etc. As an experiment, I've made her both younger and older. In both cases, her head texture becomes significantly different in colour to her body. They look fine in the preview window, but only in game does the problem appear. Is there some way to fix this?

In the object window select your NPC, and hit Control + F4. It will create a NIF and a DDS that should fix your issue, unless your NPC is new and not just an edit.

Explained here
http://www.gamesas.com/topic/1343692-color-mismatch-facebody/

I think you can share your problems on the related topic http://www.gamesas.com/topic/1343692-color-mismatch-facebody/ !

Excellent, thanks both. I admit, colour mismatch was not something I thought to search for.