OK, let me tell you my horror story about wash-out temporary color. Now granted, this was over 30 years ago, so things may have changed.
Our high school was doing The King and I as our musical that year, and I was very much involved both as a chorus member onstage and assistant to the director off stage. I knew that for the three days of the performance, I was going to be very busy. I have very light blonde hair naturally, and they were going to be spraying the folks in the chorus with black, but I figured I’d be smart, and bought Clairol’s Loving Care [sup]TM[/sup] in black, which was supposed to wash out in 4-6 washings. Perfect!
I used it at home; it was trivially easy, just as the box said - shampoo in, let it sit for a few minutes, and rinse! Except that it didn’t quite take everywhere, so I had what looked like a few grey streaks. OK, fine, so I was one of the older “wives,” no problem. But they still sprayed me a little to fully darken it up enough. Hmmmn.
The next morning, I washed my hair first thing. It didn’t really quite look black anymore. In fact, it looked, well, kind-of …olive drab? Not green enough to be GREEN, but definitely not black, or even quite brown.
For the next couple of weeks, I must have washed my hair four to five times a day. I continued blow-drying my hair and styling it nicely, but it was this weird color, the closest description of which I can give you is army green. School was still in session, and I could hear people whispering behind me in the halls. So finally, I decided to take action. I marched into the store, and bought some blonde hair color, to get me back to my natural shade, or at least as close as I could. The result: *light * Olive Drab.
My mom finally took me to the local beauty parlor (it was a small town, so there really was only one), and they, working from memory of my original hair color, were able to give me something close to my natural color again. It took over five hours of bleaching to get that color out. Apparently, as they put it, I had “porous ends.”
Now, the chances of this happening to you are slim. But NOT non-existent. I know the idea of coloring your hair for the performance is appealing, but I recommend you stick to the absolutely temporary stuff, that’s sprayed on and is simply a coating that will wash right off. If you want to go blonde, do that as a completely separate event, with no association with this performance or the dye for it. And yeah, you’re probably best off going to a pro. I don’t think you need to go to a high end place; you’re talking highlights for fairly short hair here - there’s not a lot of potential disaster that can’t be grown out pretty fast in the absolute worst case.
But dying over black dye is a very different thing; your greatest probability is that some component part of the color will come out more readily than the others, leaving you with red or green hair. And not good red or green, and not necessarily consistent across your head. You just don’t want to go there. I’ve been there; trust me on this one!