I don't think that the change of colour in your plant might be caused by lack of water.
I can tell that the change of colour for too much exposure to the sun's rays or for low temperatures , is given by the action of natural substances called "xanthofille".
They are not like carotene and vitamin A precursors involved as protectors of oxidation in many biochemical processes.
Carry a part of the report on "xanthofille" by the University of Verona, diparimento of biotechnology:
The "cycle of xanthophylls", consists in deepossidazione of violaxanthin to anteraxantina and subsequently zeaxanthin. The latter is active both in the free membrane, where it contributes to the scavenging of reactive oxygen species, is bound to proteins Lhc where it induces a conformational change that increases the ability of these proteins to dissipate heat energy.
They protect the plant from the sun so violent and extreme cold.
The betaleine, which we talked about earlier, are natural pigments that are found almost exclusively in plants of the order of caryophillales where they replace the anthocyanins.
They, precursors of vitamin A, are present in the flowers and fruits of the plant, but not in the body, so it can not be responsible tanning we are talking about.
In cacti anthocyanins are not synthesized (or are only in small part) and are replaced by betalaine, in particular in the flowers.
The absence of anthocyanins and therefore of betaleine which replace them, it would be responsible for the lack of blue flowers in blue cactus.
I hope that my English can be understood.