Well it's worth testing. But I tentatively think needing to collide it with a Colony means that you're usually getting it late enough that gaining Platinum to your hand isn't going to be your entire strategy. Or if it is, you really have to do some work setting that up.
It weakens the card, but one simple option is "gain a card costing less than the discarded card (but not more than $7)."
Without the self-trashing clause, I think that Platinum/Colony games would often have a strategy of: Build deck that over-draws itself, gain at least one 1 Royal Foundry and 1 Platinum. Discard that one Platinum for Royal Foundries. Then repeatedly discard the same Platinum for Provinces. It wouldn't be about discarding Colony for Platinum because there's no need for more than one Platinum (and buying Colonies with Platina would be too slow, particularly in games with 3+ players where the other players would empty the Provinces faster). That said, I imagine good execution mattering a lot; it would definitely reward skill.