I read a few comments about this to the effect of, "Well, maybe the artist slipped Cass in and some editor or higher-up at DC said the colorist had to change things so it wouldn't be her." This has happened before.
Artists do slip in little background jokes, parodies and commentary. Some see print and we get a laugh, or else they become infamous, such as the ones that are insults or criticisms aimed at other companies or professionals. Sometimes the legal department gets into the act when some extra in a scene violates another company's trademarks or other touchy issues that might lead to cease-and-desist or litigation. Or a well-meaning artist drops in a small tribute to a beloved character without first clearing it with the company. For whatever reason when the people involved deem a character's appearance inappropriate, the word comes down to modify the art. Some lines added here, some subtracted there, one or two colors swapped for one or two others.
That's certainly a possibility here. The artist drew Black Bat Cass and DC balked and said, "Bleach her hair and add some purple or something on there to make her look at least a little different."
On the other hand, there's a difference between some small background element or dialogue-less extra in a crowd scene and a character with some word balloons attached who appears in multiple scenes. If you have a character talking and interacting with other characters, more hands have touched the material beforehand. Meetings or at least emails between editors and writers, writers and artists and all that kind of collaborative jazz. Character design sheets passed back and forth.
No one's working in a vacuum here and it's not as if the writer and artist simply get together at Starbucks, make a comic and then mail it off to the publisher with a note saying, "SURPRISE!" At least I don't think they make comics that way. That would be a very cost ineffective way for a Time-Warner company to work, especially in these days of corporate narrative control with all these planned crossover events and whatnot. You might get something that completely doesn't fit and have an entire domino-effect set of changes throughout the line, wasting time and money.
I could be wrong-- I have been before-- and I don't mean to suggest an anti-Cass conspiracy or any kind of insidious intent. I think it's more likely this is meant to be a Black Bat that's not Cass. Or, if it's meant to be the return of Cass herself, it's simply been botched. Which would make me sad because I'd love for her to make a triumphant return in a recognizable form rather than have us stuck with a Cass who is about as un-Cass-like as any of those latter Cass Cains that got us into this mess in the first place. Because I don't see any form of Cass going from silent fighter to dyed-hair chatterbox in a mere five years. This new character could turn out to be plenty cool in her own right, or she might be a one-off they kill at some point and we never see her again. Either way, she's just not Cass.
And one or two have mentioned that the purple costume elements and the character's dialogue remind them of Stephanie Brown. I don't know. I'm not reading these things because there's no Cass, but I could see a future scenario where Steph takes up the Black Bat costume rather than Cass, especially if there's no Cass and never has been one. After all, Steph took the Batgirl costume post-Cass. If the DC universe needs a Black Bat (and that's debatable), then Steph would be the most logical choice for filling that role at some point in her crime-fighting career. I think this is a pretty neat idea, but I also don't think this character looks that much like Steph, at least none of the pre-New 52 images of her with which I'm familiar.
Anyway, I'm about as far out of the loop as anyone can be, so it could be all these points have been addressed within the story or some interview and I'm as wrong as can be about all of it. That's cool. For the record, as one of those vocal Cass fans you hear so much about on the television news, I'm not angry about any of this. I'm not out 2.99. Not picking a fight. I'm just happy to have something Cass-related to think about way out here in space. Although drawing endless Cass sketches is fun as well.