The initial process in developing better values in your paintings requires a precise focus on a couple of crucial skills... the ability to visually identify the darkest dark features in any subject... followed by the whitest... lightest... brightest features.
Once you've studied your subject... looking for the darkest darks and then... the lightest lights... everything else falls somewhere into the middle values.
And... you'll clearly begin to understand how we tend to use middle value colors to paint about 75% of our paintings! Therefore... if indeed you are trying to improve the quality of your paintings... you'll find the fastest route right there in your middle values! The darkest dark brushwork you put into a painting is indeed crucial. But as you can see below... paintings with just darkest dark brushwork don't work very well. Look at what happens when I separate the darkest brushwork... from the middle values.
The darkest dark brushwork and the lightest, whitest, brightest highlights... are the stars of our paintings. However... it's the overall middle values that set the stage and establish the overall mood by providing the eye with a natural continuity between the darks & the lights. Without the middle values, as seen above, there is no emotion. The mood is lost in stark contrast.
I created this graphic to help artists identify values better.