So, as per Where can I find specific dye colors? it seems that dye colors are somewhat random as to where you find them. In Act II, I was able to buy acquatic dye and spring dye from the merchant to the right of the entrance to the palace (the one with the pipe).
I am creating a set of armor that is colored teal with some spring green accents, and I bought 3 spring dyes from him over the past few trips I've made to town. However, now it seems that he no longer stocks the spring dye.
Is this temporary? Will it come back?
As a corollary, what is the determining factor for when merchants cycle their goods? Every time I come to town? Once an hour? Once every set of small bosses I defeat?