So just like any listed company you can buy and sell shares on the stockmarket, this is good for those times you need cash for upgrades or if you want to invest your hard earned money, you would get dividends etc, the gdp would effect the shares of each colony and you could track yours and other colonies on a share market, which is some new type of building.
Could also have takeovers of other colonies when you own a majority of shares, no idea how that would work but it could be intresting.
Just another layer too this awesome game.