If you buy the D-ticket (this is the one that you exchange for D-medals and gives you 7.5% rainbow/92.5% chances) and get a gold from it, you gain 30 rainbow medals.

When you have 300 rainbow medals, you can exchange it for any rainbow you like. The rainbow list will be kept up-to-date, so you can get future characters from it.

If you bought D-tickets and got 5* characters from them in the past, rainbow medals for them will be handed out next week.