John Singer Sargent (1856 - 1925) was an incredible watercolourist, and a man ahead of his time. One of the things I love about many of his paintings is the exquisite way in which he handles light. Just look at the two examples of his work below. Doesn't the sunlight just blaze out of them? And he makes this happen by his smart use of colour in his shadows.
Even though both the ships and the ox are white, he brings amazing colours to them by the way he handles the shadows. There is colour everywhere and the paintings 'zing'. He has been clever in his use of complimentary colours from opposite sides of the colour wheel, with the blue and the orange ochre bouncing off each other - always such a good combination of colours. But more than that, his lively brushwork and confident mark making, putting just the right colour in just the right place, tell of a man at the top of his game. 
Shadows do not need to be boring! Next time you paint (and why not even have a go at copying these paintings here?) liven up your shadows with bold colour, and see what a difference it makes!