I don't think so. It appears that all that matters is the level of Seth's pyramid. So, if Seth is corrupting a pyramid, he can buy lower than or equal to the level of his purple pyramid - even if that level is unavailable for the Player that controls the pyramid. I refer to an answer given in the consolidated FAQ:
How does the Night Purchase tile work? So, on reading the rules (I agree they are vague), there is no mention that he can only buy at the level his opponent has the potential to buy, and this answer seems to clarify the matter. Disclaimer: I don't know who gave that answer however; so I don't know whether it was a designer or not. Edit: I found a thread where one of the designers confirmed: Once Seth has a Level 4 Pyramid, he can buy ANY available power tile* I know that when I read the rules I thought to myself: So, if I'm playing as Seth, I want a Level 4 Pyramid ASAP. This is just one of the game-balancing advantages Seth has.
Seth can buy a tile of any color. He resolves his power before the Alliance, including a Night power on the same Night Phase. Once Seth has a Level 4 Purple Pyramid, he can buy any available Power Tile in the game!
*This was something I meant to mention originally - there still needs to be a pyramid of that colour for Seth to corrupt. If Seth uses the yellow player's action token to corrupt a pyramid, and the yellow player has three pyramids: blue, white, and black; Seth cannot corrupt a red pyramid. Furthermore, for example, if NONE of the alliance has a red pyramid, Seth cannot buy any red power tiles. That's one way of stopping him getting Initiative
Also, when I say Seth can buy "any", I don't mean the alliance-exclusive tiles which Seth is not allowed to buy.
However, the converse is presumably also true: If an Alliance player has three level 4 pyramids, that player can buy any remaining purple tile; even if Seth's pyramid has not been raised to Level 4. This instance is highly unlikely but theoretically possible.