Red and purple slate are found on the Vermont-New York border of the Slate Valley. The colors are due to the most common element on earth- Iron. Iron occurs in its natural state as iron oxide, or iron ore, and these also in combination with other rock formations, in this case with slate. There are many forms of iron oxide, ranging in color from yellow, brown and red to black. Each color is given by the addition (in their formation) of trace elements. In the case of red and purple slate, it is Hematite which colors the iron oxide. 

The slate containing a prevalence of this oxide can range from red to brown and is extremely hard and unfading. It is also difficult to quarry and cut. It occurs in complicated rock formations, and there is rarely large sections of clear slate. Flint joints often interrupt the slate formations. When it is quarried, the slate is sometimes too hard to cut, and ruins the diamond saw-blades. 

Due to the difficulty of production, red and purple slate are very expensive and usually used as highlights to decorate green slate roofs. If you look around, you will notice that red and purple slate roofs are infrequent, and usually made with small slate. Now you know why. They are very long- lasting roofs, however.