Watercolors have been a key medium for many 19th century landscape painters, enabling them capture the fleeting colors in nature in the field. But, their watercolors weren't the kind typically introduced to young children in tin trays. Those sets tend to produce paintings that are pale and seem washed-out. No, theirs were of heavy pigmented mediums, that they probably mixed themselves. Such concentrated paint enabled their watercolors to reflect sharp contrasts and deep hues of a sunset or sunrise. And, being water colors, they were easy to use in the field or where ever they could find water to work the paints.  JWHTurner, frequently painted on surfaces where a wet brush could later remove a layer of paint, providing sharp contrast that would otherwise be difficult to paint. Watercolors can indeed be a creative medium to work with in or out of the studio.  

Here is a study of a work originally by Fredrick Somers, that I constructed mostly with watercolor (I added highlights of greens and yellows in oil). 

12"x16" Watercolor and oil on aluminum 

The watercolors were applied on a layer of light acrylic on composite aluminum. Painting with these three mediums in this way, allowed me to complete the work in a single sitting.