As far as I understand (could be wrong), Chromas for legacy skins from events are available for purchase for RP during the event and then become available again to purchase for RP at future events of that type in subsequent years (so chromas from 2018 snowdown will be available again during 2019 snowdown).
So they are not available either for BE during essence emporiums, or for event tokens during subsequent years events , only for RP?
I feel like they should be available for purchase via one of those formats. Because my situation is like this: I bought a legacy skin for RP from "Your Shop", I now cannot buy chromas for it except during the next snowdown, for more RP. On the other hand I have non-legacy skins I have received for free from hextech chests, for which I can buy chromas for BE during essence emporiums, doesn't seem right - bought skin, gotta pay even more for chromas, free skin, can get chromas free too.
So I think that, once you have a legacy skin, the chromas should be available for you to buy for RP at any time - and then made available for Blue Essence during Essence Emporiums.
If not they should at least be made available for non-RP currency (blue essence or event tokens) in subsequent years events (obviously they would be RP only when they first come out).