This is an exercise I did in my watercolor class recently. In the classroom, a slide of an unknown painting was displayed upside down and very blurry.
First, watercolor paper was soaked in water and placed on a nonabsorbent board. Only three paint colors were used: ultramarine blue, bright red, and bright yellow. These three colors were mixed to obtain other colors. The various values were obtained by using more or less water with the paint. It was also possible to partially remove or "lift out" an area of color with a clean wet brush.
The procedure was to copy the composition, colors, and values which were displayed on the projection screen. As we worked the focus was increased, and the paper dried gradually. As you can see from the blurry images below, it was impossible to tell what we were painting until nearly the end. As the screen image changed, modifications were made in our paintings until finally a clear image was displayed.
The painting is Water Pitcher by Vermeer--I found a copy at the Web Museum that I am using here. The original painting and my watercolor "copy" are displayed at the bottom of this post.
The Web Museum is a good place to view and read about paintings by famous artists (look for the page with famous artworks or the artist index). It also could be used as a source of paintings to copy. Most of the images are large enough to see details. If you are working at home, you can use the image on your computer without need for printing. If you are viewing the painting directly from your computer screen, it will look brighter and clearer than if you printed it.