From:
Kaufman, A.J. and V.I. Lohr. 2008.  Does it matter what color tree you plant?  Acta Horticulturae 790:179-184.

Abstract:
Nursery and landscape industries produce and sell many varieties of trees throughout the world, yet little is known about how plant characteristics affect people. In recent years, research has begun to reveal that people respond to plants in a variety of ways, such as reduction of stress and increased metal concentration. A question that often arises in the nursery and landscape industry is “does it matter what color tree I plant”? This paper discusses a research project that examined whether people respond physiologically to trees of different canopy colors. Results indicated that all canopy colors were calming, but green colored canopy trees were the most calming and realistic looking. This research supports earlier plant-people findings that plants generally reduce people’s stress and suggests that particular tree colors evoke various physiological responses. This information can be a vital tool in production, marketing, and landscaping with trees of different canopy colors.