Spot on Magnus!...a 'pinch' of suppliment shaken over the crickets before feeding is actually a dosage many times the equivalent of the RDA for many of the vits in a human..unless you're going to use one pinch for literally thousands of crickets. The other problem is that the suppliments are not designed for Phelsuma specifically..They're often just a regular suppliment for birds/fish/domestic animals which has been tweaked with reptiles in mind. In the absence of specific data, the mixes used for other animals, which have been designed with some scientific basis, are probably a reasonable starting point, but the tweaking, probably with good intentions, tends to distort things. For example, turtles appear to need higher Vit A...so some 'general reptile' suppliments have got vit A that is out of proportion with other vits (notably D and E). Some reptiles 'need lots of extra calcium' so the suppliments have vast amounts of calcium added. Excess calcium prevents the uptake of other minerals (zinc, magnesium etc) as well as causing hormone feedback problems. Potassium excess assists in calcium uptake, so there is one reptile suppliment that contains a ratio of potassium that's about 1000 times what it should be (in a human RDA ratio). Calcium uptake may be better but what about the calcium/sodium ratio? The calcium/phosporous ratio should be somewhere between 1:1 and 2:1 (working on the ratios in other animal groups). Whilst the Ca/P ratio in crickets isn't perfect, the ratio in suppliments is often much too far the other way. The right ratio is the important factor...you're not trying to drown out the phosporous.
I'm not saying that there's no place for suppliments, just that they need to be used sparingly as Magnus says. Its not easy to know which one to use. Perhaps the best approach is to use one brand one week and another brand the following week. I make my own from scratch as it enables me to look at the effects of other additives. The question of how much is safe is a hard question to answer. Obviously a fair bit is likely to be cleaned off the cricket if dusted on. You can guage it a bit better by mixing it with the fruit..but even then, with something like vit A, its not only about the intake but also the uptake..which will depend on the diet of the gecko.
...and back to the original question!...There's not a huge amount of readily available published research on the causes of pigmentation in geckos. Good quality light can help you get the best out of the gecko at that particular time. Balanced supplimentation plays a role. I suspect that carotenoids may be involved in the reds/oranges but simple supplimentation with these has no effect. In a roundabout way, I suspect that diet is the answer, particularly in the case of blue colouration...'course I can't prove any of this yet!!
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