The simple answer: blackish/brown = poo, white = pee
Technical answer from the gecko spot website:
Feces can differ in consistency depending on the type of food that the gecko is eating, and the amount of water it drinks. However, feces which are watery, loose, contain undigested food, or particularly if they contain blood or are green are indicative of a problem. Other symptoms which can be associated with abnormal stools are lethargy, parasite infestation or a bacterial infection. Geckos with abnormal feces should be taken to the vet, ideally a fecal sample should be examined, by the vet, for signs of parasites or bacterial infection.
good question, I was kind of wondering that myself. mine is normal then brown/white. I was concerned about the white stuff in it and was going to make a post and find out. Anyhow glad that we found out.