Hi!
I can explain that!
Gold Keys were used to open Crates #1s ( I wasn't there but I believe stitch, princess minnie, and green flips were in them ) but I heard stitch and minnie are no longer in them and the gold keys are worth a TON so I don't recommend trying to trade for those.
Silver Keys were from the Christ Ornament Crates ( which have the gingerbread benches, gingerbread tree, toy soldiers, wreath ) but those are not very valuable and you can probably trade easily for those!
Heart Keys were from valentines day to open the valentines day crates ( had heart chairs and Enchanted Roses ) not sure on those values but the rose was the most rare item in them ( I think )
and Green Keys were from HOST Events consolation prizes ( when you lost during the event time ) but the crates are not out to open them yet!
Hope this helps!