What color/colors would you make this? What converted mana cost?

The first ability is blue or white, the second is black or white, and the third is white. Cost it as ? Maybe just since it's triple-colored.

The first ability makes me want to design a similar Painter's Servant walker like:

+3 - Same ability.
-1 - Choose a color. Each opponent sacrifices a permanent of the chosen color.
-9 - Choose a color. Your opponent can't cast spells of the chosen color for the rest of the game.

And that'd definitely be White/Blue in my eyes. Your design has that slight wrench in its gears due to the use of -1/-1 counters, otherwise I'd definitely pin it as White/Blue. I'd probably cost it as , if I weren't going to change its abilities instead.

I don't know why people think color changing is white... white has a history of color changing, but only changing things to the color white.

But that last ability is much more of a white ability. I think M Houlding got the cost right. Alternatively, you could go with , since changing color is a green ability as well... though it's normally reserved for things that change their own color, I can squint and see it on a planeswalker.

I really like that idea behind the ultimate. It kind of feels more like an epic sorcery going off every turn, except you can still play other spells (Epic, like in Endless Swarm). You'll probably want to add reminder text saying (This effect does not end), since many players wouldn't realize that at first or second pass. Also, you may want to add some other sort of boost with the protection, since "Gain an emblem with 'All permanents you control have protection from the color of your choice'" sounds like a normal ultimate, and this, of course, is much worse than that.

Edit: Hmmm... I'm looking at that second ability and realizing that it could, theoretically, be perceived as a white ability. White doesn't normally deal with -1/-1 counters, but who's to say this planeswalker hasn't figured it out? And, you know, you can always site Infect for precedence.

I'm just saying that if you made this or , I wouldn't complain. I would also accept some funky hybridization like , but now I fear I'm getting too weird.

You? Too weird? Never. :P

For the record, infect is a green and black ability. Don't cite Lost Leonin and Razor Swine as precedents; that's like using Time Spiral block to determine what color to make an ability.

I agree that the ultimate feels pretty epic.

If you let me get away with it, I would site Witch Hunter as precedent... :P

Though, I should at least clarify my comment. I wasn't citing cards like Lost Leonin to justify why white can pass around the -1/-1 counters. That would be in poor taste. I meant to point to infect as a "Wizards doesn't seem to have a problem with giving a card a mechanic that isn't in color as long as it supports story and/or supports an already existing mechanical identity". In this case, we are giving -1/-1 counters to a creature for having dealt damage. If you remove the -1/-1 counter part, that is very white. The idea that a planeswalker from another part of the multiverse, like some world where they use -1/-1 counters to represent damage, using -1/-1 counters to punish creatures that go around slapping other creatures... well, that's very white, even if the mechanic doesn't fit perfectly in the color pie.

I'm surprised that you would suggest green, because that's the last color I would have put her in. She doesn't feel green at all to me. I agree that she should be either white-blue or white-blue-black.

This was a fun discussion to read. Maybe I will do this again... or make this a design challenge.