My pregnant guppy is pooping yellow, then black, then yellow poo; is this normal?
Is it a sign she is going to give birth soon?
This is normal; you probably feed flake food and this does often have multiple colors. This is a trick the producer uses to make you buy it - however, it is not different content in the flake food - it is the same content in the food, only the coloring differs.
The coloring is useless for the fish and is not absorbed by the fish, so it passes into the poop.
When the spot around the anal area of your guppy is almost black your guppy is ready to give birth; but be patient, it can still take some time before the fish gives birth, maybe several days (this is one of the things we have no control over).