Coloring Christmas Trees Before Harvest

For some species, such as Fraser fir, the deep green color can be maintained by adequate levels of soil fertility. The application of nitrogen fertilizers on a regular basis is sufficient to maintain healthy trees with the desired color. Many species, however, are not so responsive to fertilizers, and for these, the most common way to maintain the desired color is to spray them with a colorant.

The link address is: http://www.ext.vt.edu/pubs/forestry/420-638/420-638.html