When I start a painting, I often do a value study. This is simply a monochromatic version of the painting which lets me know where the darks, mid tones and lights should go. I've found the value study greatly enhance the success of the painting. Value studies familiarize me with the subject matter, the drawing and often the contrast that results increases the depth in a painting.
On the left you have the Value study of a rose I wanted to paint. I started with the darkest darks and proceed to the lightest lights.
I was working on a toned ground with a pale wash of yellow ochre so that the yellow is my lightest area. You can also have the toned ground act as the mid tone, burnt umber as your darkest area and white as your lightest value.
Once the value study is done to your liking, let it dry completely and then go over the values you put down with color. To the right is a finished painting with the colors painted in to create a finished painting with color value and depth. It is very Important to have a range of contrast in your paintings from the darkest to the lightest to truly show off the colors.
With a value study the dark areas are usually darker than you would have logically done without it. If you squint your eyes you simplify large areas making it easier to see mid tones, light tones and shadows.
If you are new to value studies try adjusting your photograph to black and white and use it to paint your value study before adding color. I know you just want to jump in there with colors at the get go, but doing a value study will make sing!