Always use the Colonize button (or the planets screen - upper row to left) to send a colony ship until you get the hang of the game.
However - know that some of us - including the veterans - sometimes forget to do this.
A good idea (IMHO) is to add a cargo pod to the Colony Ship especially those headed for breathable worlds. When you click Colonize - SE will load all the pop the ship can handle - gives you a kick start on pop. Remember - the more rabbits you have - the more bunnies you get.
I have 3 colony ship designs/atmosphere:
- LR - long range (has a supply pod) to grab those artifact (ruins) planets
- SR - short range - with cargo pod for in-range breathables
- SR-nbr - no cargo pod for in-range red-plus planets.