There are many shades of white in this world. Snow white is what I have been looking at for three frigid months now, but I dream of shades of white for spring and summer. First come the snowdrops - as white as snow. A very welcome white. Rhododendrons bloom towards the end of May, but 'Boule de Neige'  (Snowball) has a memory of the white winter. Somehow this pristine white seems prettier than the snow. High summer and…