Sun tan effect?

I have noted a strange thing happening with some of my figures today. Most of my single figures are, as long as I neither own an appropriate showcase nor have the belonging sets finished, stored in a plastic box which is stored in a closed cabinet.
Today, I took out that box because I wanted to have a look at some Naples light infantry figures which I have painted some months ago.
And now here's the strange effect: some of them look sun-tanned, but in a slightly orange way. :-(

When I put them into the box, they looked like this:

And that's what I saw when I took them out of the box today:

They represent an earlier stage of my work. But as there are two other Naples light infantry regiments left to paint, I wanted to refurbish them in order to mount all four on the same mini base.
Now look at the flesh colour. I precoated with GW FortressGrey, painted the faces in GW ElfFlesh and made the contrast with thinned GW SnakebiteLeather. A mistake, as I see. The whole composition seems to become orange-brown after a while - and even without the effects of direct sunlight.
Does anyone else have noticed similar effects on his/her figures, too? Or is it just the fact that the ElfFlesh layer got in contact with a brown colour?
My cheap hobbyshop acrylic has shown no such effect.

Looks like I have to refurbish some older figures' faces. As if I hadn't anything else to do, eh?
I'm currently preparing a group of guard grenadiers of the Kingdom of Naples in early uniforms. They will very well go together with the grenadier flag group from the Strelets dismounted staff officers' set.

Apart from that I've prepared something for the next painting competition on Bennos' forum - my baby son gave me two terrible nights with very few sleep, so I used the pauses between hugging the baby and singing goodnight songs to do some painting work. ;-)