This was a blog post I made back in 2014, but I've moved platforms and I wanted to make sure it was still out there, because it remains one of the tools I use most often - even more than flipping the canvas!
I don't know about you guys, but when I start, one of the first things I do is a 3-tone value thumbnail, to lay out the graphic elements of the painting I'm going to be working on....but sometimes, keeping track of that beautifully clean study can get tricky when you introduce detail.
Many of us create some kind of black and white mask on top, but that still has a tremendous amount of tonal variation, as well as detail that can make it hard to see what you are really working with.
Here's how I cope with this situation:
Here's an image I've loaded up in photoshop. The first thing I do is create an empty folder in the layers palette above the image.
From there, we are going to create 2 adjustment layers inside the folder. If you haven't created adjustment layers before, it's the icon in your layers palette that looks like a circle cut into a light side and a dark side.
The first adjustment layer is a "Black and White" layer. I just use the defaults. This has given us that value-range version of our painting, but we're not done.
Now, create a "Posterize" layer, and change the value to '3'. That's it! You now have a 3 tone simplification of your painting. Best of all, the posterize layer simplifies a lot of detail, so you're back to seeing things as shapes instead of details.
Because you put these two layers inside a folder, you can turn on and off the folder at any time to get a perfect view as to how the lights, mids and darks of your painting are working together.
I hope you find this useful, I would love to hear if it helped!