Hi Mack, 

Thats a very interesting observation you have made! There are some plants which have purple flowers but not yellow. One example is a very common plant grown in the southeast United States called a crape myrtle (Lagerstroemia sp.). Crape myrtle flowers are commonly purple or hot pink, but not yellow. Flowers are commonly purple because of a chemical in the plant cells called anthocyanin. Anthocyanin is a pigment that absorbs green wavelengths of light. Anthocyanin can cause red, purple, and black flowers based on the concentration of the pigment in plant cells. 

In general purple is a color that is rare in nature. Anthocyanins are more unstable than other pigments like carotenoids (yellows and oranges) or chlorophyll (green), so there needs to be a strong selection pressure for plants to evolve mechanisms for creating purple colors. Purple is a very desirable trait in flowers, so plant breeders will use methods to try and increase anthocyanin concentration in plant cells. Some of the methods they use are Ethylmethylsulfonate treatment, radiation treatment, and interspecific hybridization. Lilacs and violets have been bred by humans for a long time, at least 100 years, creating the large variety of flower colors you see at nurseries. 

Hope that helps!