For a treasure based colony game, I think you should look to get one or two platinum, then start buying colonies, and take provinces with 5-6 colonies in the supply. It will vary with each deck of course. It's also sometimes worth making a decision on greening after a reshuffle.
In money games you tend to build your deck up to a peak, capable of buying the top victory cards, and then let it deteriorate as you add green cards. By the time the last colony is in the supply the average hand is below colony quality but the variance gives a chance of colony buying hands. For your Gokodom game, the mountebanks/goons had done enough damage that improving the peak capability of your deck was going to take many turns but the deterioration from buying green cards was going to be less. The peak should have only just been at colony level, and no more, with an immediate switch to provinces as soon as you went green.