I haven't noticed mine getting smaller before molts - before my male molted the last time he actually looked a bit overfed, because his larger abdomen was all squished up inside the exoskeleton. It flattened out a lot immediately after his molt. Hard to say about the color, as green males will always turn brown and green females will too if they're kept in a dry environment.
It stands to reason they'll only get fatter as there's a larger mantis forming underneath. Most of the "fatness" consists of fluids needed to pump through the soft exoskeleton to expand its size before it dries and hardens.