Replace specific range of color in Photoshop with a flat color

Question

I wanted to be able to replace a range of color with a single flat color.

I'm mostly dealing with black and white / grayscale images.

I tried to use the Curve adjustments tool, but the problem is that it doesn't allow you to create straight lines

Here's a random simple example:

Here I want to do three things:

1. Replace all colors with values ranging from 0% to 26% with a flat color of 24% value.

2. Replace all colors with values ranging from 26% to 76% with a flat color of 60% value.

3. Replace all colors with values ranging from 76% to 100% with a flat color of 89% value.

And by flat color, I mean no gradients, smooth transitions, just a completely flat even color

And as you can see, the curve tool just does not allow me to create straight lines, making it almost impossible to achieve what I want to do

I tried to use a Gradient Map too, but it doesn't allow me to replace the colors I want with flat colors

Is there any fast and practical way for me to achieve that? Preferably in Photoshop, but if it's necessary to use other softwares, I'm open to that.

E D I T:

The type of images I'm working on is grayscaled photographs and realistic paintings

Here is an example of the kind of image I'm working with:

(sorry can't embed, not enough reputation points, see my comment below)

This is what I want to achieve:

To clarify, I don't need it to be the edges to be so clean, neat and well-designed. I'm only using this as a fast and speedy visualization tool, and not to create a final product. So speed is important

Answer 1

Use a Gradient Map to determine your break points and apply colors accordingly. Since you want it flat you'll just need to put in extra points to make hard stops.

Adding a Gradient Map, see how there's green at both ends of that color. You can't tell in the screenshot but there's also identical blues and reds at each end of those colors too (you could also remove the ends entirely and then you only have to worry about any colors in the middle):

Which creates:

Answer 2

Here's a method you might find helpful. It uses the Cutout filter as discussed in my comments previously, but in conjunction with a Smart Object and Adjustment layers, which should give you some finer control over the conversion of tones to flat areas in the Cutout, with some nice visual feedback to the adjustments as you are making them.

Proceed as follows:

1. Open an image

2. Add a black and white adjustment layer if the image is not already black and white.

3. Add a Levels adjustment layer

4. Select all the layers in the layers panel, and choose Filter > Convert for Smart Filters.

5. Apply the Cutout filter.

6. Double click the Smart Object to open it, and arrange both Windows side by side using Window > Arrange > 2 up Vertical

7. In the layers panel select the Levels adjustment, and move the sliders.

8. Each time you adjust the levels hit CTRL+S to save the Smart Object, and the Cutout filter will update in the other window.

Here's an example of the technique working.

Another nice thing about this is that the Cutout Filter is applied non-destructively to the Smart Object, which means you can go in and adjust the number of colour levels, and other sliders in the filter.

Answer 3

Take freeware. Image editor Paint.NET has plugin Curves+ which allows also piecewise linear curves.

Curves+ is a part of Pyrochild plugin package which also is freeware.

NOTE: Paint.NET and its plugins are available from numerous download sites. I recommend to avoid other than the original site https://www.getpaint.net/ because malware pushers use popular freeware to get people hooked.