it can be if left for a long time but usually its just brown
No. It is more like the color of human feces and that of other mammals.
It can be white and this can be caused by diet changes medication etc.
you aren't eating enough.
Birds do not urinate as people do. Their urine is excreted with their feces. Their feces is actually brownish-black in color and the white you see mixed in with it is the "urine", which is actually very concentrated urea, not urine itself. Also, usually the bird feces can be white with a black spot or dot, not the other way around.
it can be pink or white depends on what fish they eat
White feces could be large amount of fat being present in your stool. Although this would usually present as a grayish color, it could be a serious health issue such as "dumping syndrome".
Could be a tapeworm. http://en.wikipedia.org/wiki/Tapeworm
Because its been there for ages and has gone all crusty and mouldy
Doesn't sound like it is droppings, sounds like worms.
My green anole's feces look brown at one end and some white. I got him of January '12 and ges pooped like that ever since.
Bird droppings are white because birds don't urinate. Instead they release uric acid in their feces.
You are probably talking about lizard's excreta/feces. It is what looks like "black with white at the end of it"