Hannens Ed,
If your axolotl is eating normally and isn't show any obvious signs of stress (e.g. curved tail end or gills that pointing forward) I wouldn't stress out too much about the colour change. It has been noted on many cases that axolotls will go through colour changes or even develop patterns as they age.
However, it is also known that discolouration is sign on parasitic infections. For example, if your axolotl is the albino/leucistic type and its skin is too red or overly yellow (for the golden type) then these could be signs of a systemic infection.
Furthermore, you mentioned that you moved your axolotl into a larger tank to lower the temperature and seeing as the weather in Melbourne has cooled down considerably (I also live in Melbourne) it could very well be related to the axolotl's capillaries. This works the same in humans. When you are hot, the muscle tissues in your body relax and the blood vessel (capillaries) dilate, thus increasing blood flow and distributing the heat across the surface of the body for effective evaporation. Hence why people get the 'flustered' appearance- because the capillaries are closer the surface of the skin. The opposite is true when you are cold. The capillaries constrict to retain heat. This would work the same with axolotls...i.e. for the pale appearance in your axolotl.
Jay.