This is a lot like "Black and White" or "Monochrome", but the distinction is that for the sake of clarity, I'm not allowing anything that isn't completely greyscale. This is easy to test for because we simply won't allow any pixel in the final JPG to be composed of anything but R, G, and B values that are not equal.
I started this challenge out to be "Monochrome". But it quickly became evident that my understanding of what that meant was fuzzy. And thus, I wasn not able to get what I wanted to see in the galleries with that name for the theme.
So I've done the unthinkable this time and actually pulled the rug out from under everyone and changed the theme after the challenge began. I don't think that's ever been done before.
I also don't think that Black and White has ever been done before either, and that really surprises me.
This one's for everyone, of course, but especially for those who have longed for a challenge theme that emphasizes a technical aspect or technique in photography instead of a subject. You know who you are!
And this theme will also now satisfy everyone who longs for a theme that is very clear cut and won't leave any room for interpretation of the theme. Photos will be on-theme or they won't be allowed. And we can all concentrate more on making good photos and less on the technicalities of whether or not a photo qualifies for the theme.
I'm really sorry to change themes like this, but I see no clean way out of the direction things were going other than to do this.