Back when I was studying watercolor at the American Academy of Art with Irving Shapiro, one of the assignments that I thought was really helpful was limiting the number of colors you would use in a painting.
WE would use one color the first week of classes and 2 colors the 2nd week of classes the 3 for the third week and full color for the final week of the month.

This limited palette use in the beginning of the month would really help you a lot to see the values in the painting and as you added colors to your pallet for each week's particular painting you would still be looking at value patterns but now would see different colors in each of those values.

For beginners, I tend to tell them to choose a limited palette, like 1 to 3 colors and this helps them a lot with looking more for the value than the colors. Beginners tend to try and copy all the colors of a photo exactly as they are in the photo and never really look for the value pattern. By using a limited palette it forces you to look at the values of each color and this helps you to not just rely on only what you see in the photo. There are many times that you will need to change things in a photo to help the value pattern work better. 

 

Start out with painting a lot of one color paintings and gradually add in a color and then later another. It also will help you learn how to choose colors that work well together.

It is also amazing to see how many colors you can get from mixing 2 colors two colors together.

 

One-color value sketches are also good for you to do before you do a full-color painting.
Give it a shot, it helped me so much when I was back in art school