I considered this working several different ways:
1. Choosing a color instead of sacrificing a permanent. However, this would mean the ability would have to cost more mana.
2. Discounting your own spells in addition to marking up your opponents.
3. Taking the additional cost from the mana color spent to pay for the ability.
This is how it ended up, for now, but it will probably change, especially once I get some comments.

Um, wow. This looks like it'll all too often be "Target player cannot cast spells" ... and what happens if you use a colourless permanent? (Or, with current wording, a multicoloured one, but that's just an obvious wording fix)

It took me a while to figure out what Vitenka was talking about. Then I realized the color was coming from the permanent put on the bottom of the library. So, if you pop out a Soldier Token per turn with Mobilization and activate this, my spells now cost an additional ? Even if I'm playing Jund? Harsh. This looks like it needs some sort of escape clause.

By the by, congrats on making this put the card on the bottom of your library instead of sacrifice. It would have been too easy to recur some silly zombie or other. But, yeah, token creatures. Even if this said non-token creatures, I'm sure something else would be too easy to keep returning. Nissa's Chosen with an uncontested Nissa Revane on the battlefield, for example. Have you ever noticed that Nissa's Chosen sounds like a cookie?

Vitenka, that's exactly what it's meant to be: a sort of Iona, Shield of Emeria that requires a sacrifice every turn you want to use it and works better against multicolor decks rather than monocolor.
I just jotted this down really quickly, so the mana costs aren't really indicative of what they should be. They should be much higher.
With the current wording, won't using a colorless permanent just not do anything?

Wit the current wording, indeed, a colourless creature won't do anything. I think you typoed, though: to me this looks better in multicolour, against monocolour.

Oh wow, now it's "Of each of" that makes multicolour very very strong. (I was assuming it was 'one of' which makes colourless, well, impossible to do, since I can't pay a colourless colour.)

This locks down monocolour completely. Ebil.

Oops. Correct you are, Alex.
Vitenka, are you saying this is more evil than Iona?

On a side note, when I went through the first stage of the great designer search 2, one of the essay questions was "Name a card currently in Standard that, from a design standpoint, should not have been printed. What is the card and why shouldn't we have printed it?" I wrote in Iona, then went on a tear on that card. So, yeah, I'm not an impartial judge on this mechanic specifically, since I've already gone on the record that I don't believe effects like this should be in Magic at any casting cost.

That being said, if I was to agree that this sort of mechanic was fine in Magic, then I'd say you're clearly on the right track. It's probably a fair implementation, it's just something I personally don't like.

If you can't tell by my designs, mean control effects are some of my favorite effects in Magic... so I guess we're on opposite sides of the fence. I would ask why you don't like it, but I think I understand.

Yeah, Iona should have been printed as "Win the game". FAR less annoying. And she costs nine. Control is fine. Control at the level of "Skip your turn, there is NOTHING you can do against this, even if you top-deck perfectly" isn't.

So - this thing at least gives you an instant speed window; and they can only use it during either your turn or during their turn.

It's just going to be a very slow and irritating lock.

Yeah, I think we understand each other. On the flip side, I'm a big fan of cards like Naked Singularity. I don't have a problem with prison cards per se, I just have a problem with prison cards that I don't have a chance of escaping. I think part of my problem is that I like it when my opponent is winning. I build my decks with the ability to win 40% of the time, so that the opponent gets to win 60% of the time. It's not that I don't enjoy winning so much as I enjoy fighting against the odds, and, since many other people do care about winning, I push the numbers in their favor. When I confront a prison prison deck, it can be a horrible experience. I play nice host and continue the game long after my opponent has clearly won, because I don't want to deny them the killing blow. So I don't concede, and have to sit in a state of suspended boredom while my opponent finds an answer. In the best case scenario, this is boring.

What's your opinion of being on the opposite end of a firm lock, Link? Let's say an opponent has a recurrable Mindslaver deck and can Mindslaver 4 rounds out of 5, but has to give you a turn every now and again. Do you handle those games with grace? I'm not trying to make a point or anything, I'm just curious.

Oh, I hate it, and my friends don't like playing against some of my decks. They won't even let me play my Gilder Bairn, Cytoplast Manipulator, and Leech Bonder deck, and it isn't even that good. My point is, I get that cards like this will only ever be fun for one person, but I still think they should exist... just very sparingly.

Heh. Now interestingly, cards that are only fun for a small subset of Magic players, and only need to exist very sparingly, actually don't tend to go at mythic. They're much more normally rare. I assume this is because the mythics in a set are higher profile, often the selling points of the set.

Although, as we now know from Archangel's Light, even mythics sometimes get tossed in at the last minute.
I made this mythic basically because Iona is mythic, and because I don't think this ability should show up very often at all.