Hello people, back with another idea. It comes directly out of the sim city playbook.
A new colony should have the ability to take out a loan from their commonwealth when they need it at the commonwealth's approval. When the new colony takes out a loan, they and their commonwealth would also have to agree on the amount of payments and the interest rate that the loan would have.
After the loan is taken out, the player can contribute to the loan at will, or they can wait until they are required to start making payments. When it comes time for the player to make payments, the payments and interest rate could either be added to the colony taxes amount to make it simple for the lendee, or they could be on their own separate bill. I wouldn't worry too much about trying to calculate what portion of each payment is interest, just have them make flat payments with an added interest fee based on their interest rate of the loan.
The interest factor of the loan is purely there to motivate commonwealths to loan money, because they would receive more money in return, and you might have some commonwealths that turn into loan sharks for the sake of making money. If a commonwealth and it's loaning colony have very close ties or the two players are very close or even relatives, the interest rate could be set to 0, but I would still have it for the benefit of the lender.
In fact, what if we could make it so that one commonwealth can loan to another commonwealth through the use of the embassy? It would allow commonwealths to make money by lending to other commonwealths, and some commonwealths could even become loan sharks for the purpose of making money.