Buying/Harvesting huge ammounts of the cheapest spice (brown), fly around the colonies of your allies and check the prices for them. You can easily find planets which buy them for 5000-8000 a piece. Now when you have 30 of them onboard, that's already a colony paid.
Ich geh nie mehr, Ich bleib für immer hier! Ich geb 'n Scheiß auf euch Punks weil ihr mich nicht interessiert!
The whole market at the space age is pretty dynamic, you will find places that only will buy your spice at only $200-400, on the other hand it can go upwards to about $10-15k on average a per unit at same places. Also if you're lucky you can sell it for $40-45k is the top ive seen per unit, but that was late in the space age in some pretty desolate areas.
So good idea is save up your spice, and constantly check selling prices.
Not sure if its the same for each play-though, but first time, red spice I over abundance of and was the cheaper spice, while purple and blue were the big earners, the only colour I didn't get my hands on was pink, just because it was pretty rare in my game.
once you get a few colonies well built money will start flowing from spices
its important to make colonies with lots of spice T3 once you are able to afford it, and once you can buy a spice storage building give each colonie one, that will give you 30 spice storage and around 300-350 spice per hour(more if you dont care for happines), then search for colonies that will pay at least 10.000$ per spice(unless you are at full stock and can carry or produce any more spice of that kind), its not hard to find colonies that will pay from 15000 to 25000 if you search a bit