Ever see those images that have most of the image as B&W with one or key elements in color? Or an image with almost unbelievable colors?
There was a technique back in film days, where we printed B&W and then hand colored certain elements. Now, we can do this digitally.
Some image manipulation programs programs allow you to paint in a color on a layer or an outlined area. Other programs allow you to desaturate all the colors except for what you want to be color. Check your programs help section or a tutorial to find out how to do this in your own program, several are very different from each other.
Even with the neat effect, the subject matter still has to fit the technique. Maybe lips and eyes for a portrait, rays of sunshine in a landscape, or a flower popping out from the background. Or even an old car in a cityscape. Get creative!
Using many of the same controls, we can enhance, or even change, colors selectively with a color image. Trees not yellow or red enough? Change it. Need a bluer sky, or even a different color sky? Change that, too.
How much depends on your vision for that image. You can keep it fairly realistic, or "interpret" it as you see fit.