The American Rhododendron Society has a website that has descriptions of most named varieties and photos of many. For the white one with the purple throat, I suggest you look up Sappho, or Calsap. Similar blotched plants include the pink Mrs. G. W. Leak or Mrs. Furnival or Furnival's Daughter and the lavender Blue Peter or Blue Ensign or A. Bedford.

The link below is the ARS website. When using the search by name, use just one word, for example for Mrs. G. W. Leak, just use Leak. If you use more it must be exactly "Mrs G.W. Leak". Perhaps this is the way the name is registered.

Two of the most striking of all plants are Mrs. T. H. Lowinsky which is white with a yellow-brown blotch and Sapporo which is pale purple fading to white with a purplish red blotch.

For a less sterile description of the plants but fewer photos, check out the site for Greer Gardens in Eugene, Oregon:

http://www.greergardens.com/

Here is a link that might be useful: American Rhododendron Society Search Site

Your favorite white Rhody with a purple throat is probably Sappho. It is a rather open grower while young.

My favorite is called Lem's Cameo. See link below for a picture of one in my garden.

Here is a link that might be useful: Lem's Cameo