A method is presented for recognizing trees with unusually colored foliage, based on the use of Munsell plant color charts. Color assessments are made by scoring the crown of each tree for hue, value, and chroma. These values are plotted in three-dimensional space for each species, and a threshold is identified marking the border of an ellipsoid. Trees with foliage outside this ellipsoid are considered as unusual. The method means that assessments of discoloration can cover a range of colors and moves the decision over the color status of a tree from a subjective field decision to a statistically defined decision at the analytical stage. It takes into account that a tree may still be classified as green (previously classed as without discoloration) when its color of green lies outside the normal expectation for the species. The method represents a significant improvement on the discoloration assessments currently being used in large-scale inventories of crown condition in Europe.