Hi Julian,
... the coloration is still kind of confusing to us too.
It definetly has alot to do with the mood. But its also an age thing...
Here is a female - in "good mood".
As you already said, sometimes the contrast between those big spots and the background increases - the smaller pigments you see on the photo seem to disappear, making the background seem more yellowish ... This has often to do with stress. But I'm not 100% sure if the big spots turn darker ... hard to tell.
It seems to be similar with the males. When they are doing fine or while spawning the contrast between they dark spots and the greenish background is lower.
It notice those blue-greenish reflections every once and a while on our female DPs. But far as I recall only when they are "low contrast"/"good mood" colored ... Actually I find some of those reflecting spots on nearly every picture of a female DPs... and I even found some on the photos of our male. Although I never saw the "blue spots" on a male wiht naked eyes.
Here a pic of a male, note the blue reflections on the tail:
Considering where those blue spots appear on the pictures and the angle at that I flashed when taking those pictures...
I believe that those pigments making up the big dark spots simply reflect light at a certain angle.
The yellow parts of the tail fin of the males ... seems to us, that it turns a bit red while spawning:
http://guppy.andreas-sander.net/Kufi_Balz2.html
In fact the thing with the low "contrast coloring" goes quite far when they are spawning. Sometimes the background turns so dark, that it's hard to see the outlines of the dark spots:

... see the reflections on some of the dark spots of the female? Little different than usual.
I' m still wondering, if its the different lighting or it the puffers pigments.
Regards,
Andy