[douglas]

If you order a colony ship to (M)ove somewhere (or warp somewhere), it will just go without taking any special action. You need to either (L)oad population onto the ship or (T)ransfer cargo to move population on board.

If you order a colony ship that has no population loaded to (C)olonize a planet, it will attempt to load population from its present location before moving. The Send Colony Ship button in the Planets screen uses the Colonize order, so it's usually not a problem if you do this. However, the attempt to load population will fail if the colony ship sent has nowhere to get the population from - this typically happens with colony ships that explored a warp point without loading population first, or when two colony ships are departing from the same location and the first one takes all the available population.

The only way to fix this once you have a 0 pop colony is to have a ship load population from somewhere and drop it off on the empty colony. You can do this with a colony ship before sending it to colonize somewhere, you can build a dedicated transport design and use that, or you can stick a cargo bay on a warship and let it do something besides fight for you. All you really need is something with cargo space (the colony module gives 20kt of cargo space, enough for 4M population).