Have you ever been blown away by the cinematography in old movies?
We're so used to little color screens on everything it just seems natural, but back then, what you saw through the camera wasn't what the movie would look like. You saw color. There weren't any electronics. You were looking through a series of mirrors and through the lens at the people on set in front of you. Cinematographers had to translate in their head what sucking the color out would do to the image... and they were GOOD at it!
They had to think in terms of luminance.
How to use luminance to sculpt your images
This is a good example of what I'm talking about. The raw image is pretty flat and uninteresting. But switching to black and white to work with luminance saved it.
I worked with eye first.
That's not good advice. Its just where I like to start because its usually my focal point and I always find it most interesting. I painted in light highlighting the bright points in the iris all the way around the pupils. the left eye is out of focus so I needed to mirror that with my brush strokes. If its blurry, use bigger brushes.
With that same light I went ahead and highlighted the highs in his features beginning to sculpt the face more. Then I countered that with the shadows painting darkness in around the iris and back out to the rest of the face. I finished with a few separate passes on the image as a whole to direct the light subtly across the eyes to add some feeling.
By this time I was getting it where I wanted it, but I decided to add the color back in.
I liked it. I compared the two a few times and yeah, I liked it.
With just a quick adjustment to tweak the color, it was done.
When I first started doing this, I'd spend all my time shifting the color around and maybe ten seconds on a quick contrast tweak at the end. More and more I find that working to get the lighting right first makes all the difference. Your eye can be fooled by color contrast and saturation. Taking it back to black and white helps you focus on what you're actually doing. Then, any color work just adds to that. This isn't a hard and fast rule, just a suggestion. So next time you've got something that's just not getting there, consider switching to black and white!