why are there platinum and colony cards sometimes? ...

[crispyP]

is this just a randomized option? is there a setting that controls it?

[jsh]

Platinum and Colony are from the Prosperity expansion. You're meant to add them to the kingdom based on some percentage of Prosperity cards appearing in the kingdom; I'm not sure what method dominion.games uses. You can specify if you want them or not when you create a game via table (in Advanced Options probably). Similarly, the Shelters from Dark Ages replace Estates in a certain percentage of kingdoms.

[crispyP]

is there a way to play (online) using just the prosperity set? i can't find a way to just play with the cards i want ...

[kieranmillar]

When you create your own table there is a button to select the kingdom cards. Here you can force particular cards, but any slots left empty will be filled with a random card from the expansions you have checked. (There is a checkbox next to each expansion name) If you have not checked any expansion, it will choose a random card from all expansions.

[Stef]

I'm not sure what method dominion.games uses.

It's the same as the suggested rule for IRL play: after you have generated your kingdom, you flip one of the cards over at random. If it's from Prosperity, use Colonies. Then shuffle the cards again and flip one over. If it's from Dark Ages, use Shelters.

Online also allows you to ignore this rule, and just set the Colonies/Shelters to a value you like. And of course IRL you can do whatever you like too.