The best way to improve your eye for color and value is by painting from life. I was introduced to plein air (painting from life outdoors) many years ago while in a workshop in Provincetown, MA. It turned me in a new direction, and I am still learning every time I venture out to paint. I've been able to incorporate into my studio work what I have learned from these quick two-hour sessions:  the color and temperature of shadows, the difference in value between the light side and shadow side of a building, the rendering of reflections. I like the spontaneity -- you have only a couple hours at most before the light changes. I almost always bring my gear with me when I travel, and have captured moments in time in many places.