How to flatten colors in Photoshop?

[standingtree902]

I'm trying to make edits with 2 distinct selective colors throughout the whole image with the rest monotone. I think this is done through color range? But I'm not sure how to make images have this effect. Example: 

If you know how to do this please explain step by step if you can, I don't have a huge amount of knowledge and have a hard time understanding. Thank you!

[Mylenium]

Apply a gradient map adjustment layer on top of your image. Of course the image itself will usually need heavy pre-processing by ways of curve and levels adjustments or even posterize filters to determine the actual contrast and define areas.

Mylenium

[c.pfaffenbichler · Accepted solution]

Please provide (a lores of) the image you want to edit thusly. 

The red »splatter« seems to be fairly illustrative as it conforms to the underlying image in some places but not others. 

Edit: