After having looked at how costumes and skin colors change according to the time of day and from interior to exterior night scenes let us go into detail analyzing a scene that takes place entirely in the dark underground but during the day.
The color reference: these are the characters' colors in natural light before (left) and after (right) the scene in the cave (screenshots further below).
Pazu and Sheeta have just landed in a cave. Thanks to Sheeta's powerful aquamarine crystal they have been slowly floating down instead of falling to their death. As Pazu has learned upon meeting Sheeta, the crystal is turning itself off slowly after its services are no longer needed. So in order not to be lost in the complete darkness he is preparing his lantern and is able to light it just as the crystal stops radiating.
The interesting thing is the relatively small degree of change in the characters' skin colors. In reality a light that changes brown to turquois would also change skin color to something like aquamarine. Skin color usually serves as a reference against which we judge surrounding colors. Since it is the one thing that stays relatively steady here, the blue-green clothes are more noticeable because our brain does not color correct the image automatically.
The background is monochromatically dark blue and therefore does not distract our attention from the characters yet adds to the overall feeling of a cold dark cave (in reality, a dark cave would hardly look blue).
But then the only light source is the small lantern which emits a warm orange light. As a result, the rocks around them and actually the whole monochrome background now look brown and considerably warmer. Costume colors are closer to their normal hues because, after all, sunlight is much closer in hue to a fire than to the crystal's radiation. Skin tones are also slightly warmer and more natural again:
|Pazu's clothes (A,B,D) and skin tone (C) in image 3 vs. Pazu's clothes (E,F,H) and skin tone (G) in image 4.|
Then they enter a wooden mine shaft. In comparison to the rocks the wood seems warmer also in shadow areas. The stronger reflections imply narrower walls.
The light of the lantern is still warmly reflected on the characters eating their fried eggs, but not on anything else:
I have omitted the part where they meet uncle Pom and just cut right to his explanations about the stones around them:
When the old man blows the flame out we have relative darkness:
|Uncle Pom's clothes and skin in the light (A-D) and in the darkness (E-H).|
All character colors including skin tone are darkened and desaturated but not much changed in hue. It is crucial that we still recognize those clothes as basically brown.
Finally, Sheeta is bringing her own glowing crystal out and the lighting changes again:
|These color swatches represent those parts that are in light: the crystal not only increases saturation and brightness of the clothes (A>E, B>F) but also affects Sheeta's skin and hair (C>G, D>H).|
Now the saturation is increased even in the shadow parts and the contrast of the light and shadow parts becomes stronger. Now the light is so powerful that even the skin tones become greenish.
|16: In the close-up we even have three layers of light and shadow.|
Then in the reverse shots the background behind the characters is dark again and their costumes and skin are seen in "normal" light again with the "plain daylight" colors. In other words: the whole séance is over and the old man exhausted.
All screenshots have been taken from this gorgeous Studio Ghibli Bluray Collection by "universum film". It seems to me that this is the way to see the film unless you have access to a 35mm print.