Pure lilacs are actually a little more rare than the lilac & tans, and they are stunning. They are created from double copies of two fairly rare recessive genes (chocolate and blue) and a double copy of a very rare recessive gene (recessive black). As opposed to pure blues, which have a beautiful blue-gray coat with no brindling, pure lilacs have a stunning silver-gray coat with no brindling, and they also have gorgeous light colored eyes that are generally blue-gray, hazel, or yellow-gold.
Charlotte and Cruz had this beautiful litter containing a pure lilac (starting at about 1:15), a lilac & tan, a blue & tan, and a pure blue. They are all stunning and quite rare.