Now that I have elemental mana worked out, I can make these work like I originally planned. Taking a bit of inspiration from Phyrexian Mana, I'm thinking of having 'coloured' elemental mana (so this would have the Elemental Mana - Elemana for short - symbol on a white mana background colour).

Yes, that makes sense. If you let this have a {EW} symbol for Elemental White, then you won't need the "~ is white" line. But the reminder text on here will be incorrect at that point. You'll want to say something like "{EW} can be paid with or with one mana from a permanent of this card's element". Which is rather less clear.

I like the "can be paid with W or one mana from an Air permanent" version of elemental mana, it's a good clean way to add versions of mana without making it insanely complicated. (I'm worried it'll be hard to find the right balance in how mcuh to spread the elements across the colours, but I assume you're designing with that in mind.)

Yeah I'm really trying to take my time and balance everything both across colours and elements.

And Alex, I don't see where the current flavour text would be incorrect, seeing as {EW} can cover several elements, if you have it on every card, spelling out exactly which element type is allowed to pay for each)

In set 2 RIFT there will be multi-element cards so the text on a Fire/Earth card would be ({EW} cn be paid with either or one mana from a Fire or Earth permanent)

also, not everything will have an elemental type - elementals obviously, Angels = Air, Demons = Void, Dragons = Fire (Mostly), etc - but as most of the more mundane creatures are made up of a balance of elements.

What I meant was that the reminder text will be accurate for this card but not accurate in the general case. I fear it might be more confusing than helpful to see two cards, both with the {EW} symbol, but on one the reminder text says you can pay it with Air, the other says it can be paid with Earth. It just sounds like a recipe for confusion, or for people thinking you've made a mistake typesetting your card.

On the other hand, people didn't seem to have trouble with Amplify being different on Daru Stinger vs Feral Throwback, so maybe I'm worrying too much.

I think specifying the appropriate element in the reminder text is probably more intuitive, but that's just a guess.

My only caveats would be:

• In the extremely rare case where a card changes colour and its non-converted mana cost matters, if there's a "white elemental" symbol (or a "white phyrexian" symbol for that matter), I assume the symbol is still white, but I think it would be unsurprisingly open to confusion.
• It may be annoying to template, assuming Multiverse doesn't yet do the interpolation for you :)

Reminder text ought to be fairly doable. Create a mechanic with text something like "_( may be paid with either \1 or one mana from a \2 permanent)_", and then invoke it with "[ElementalReminder {W} Air]".