It is very easy in Photoshop or any other paint package with similar functions. You can do this to any image you want to paint to aid you in discovering the true tones or families of related tones.
Let's start with a random image. Here is one of a Motel in L.A.
It is a not a very complicated image but it can be tricky to determine what are the prevalent tonal values just by looking at it.
Image > Mode > Grayscale (discard color info , yes, but do not save the image over the old one)
Second, let's posterize . There are infinite ways of doing this.
Posterize just removes all gradations and returns an image with the amount of levels you specify.
Image >Adjustment> Posterize (level 3)
Because it's a little grainy, let's blur it a bit.
Posterize yet again
And there you have it.The Motel reduced to grayscale. I bet you didn't guess the sky was closer in value to the asphalt.