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One other time spiders change colour is when they lose a great deal of body mass, such as when they lay a heap of eggs, they can change colour dramatically. I have pics of a gravid Araneus quadratus, sort of khaki colour, then next day a very much reduced spider that is a deep red colour.
The reason this happens is that the colour is caused by chromatophores (search the net for it, the answers are interesting if you have not come across this before). Skin shrinks, colour changes.