Planting dill seeds outdoors in Z6 right now is fine.
Sometimes young seedlings will pick up a particular color from something in its soil. Once planted outdoors, it will usually grow to a normal size and color. Mammoth Dill that is given plenty of natural nitrogen will get up to 5 foot tall. Mine drops seeds from last year and they sprout in the same area every year, unless I pull them up before they send out seed heads.

I don't know what to tell you BUT LONGWOOD GARDENS outside near their fountain show has containers of what appears to be purple dill. Deep Purple. Tastes like dill with anise overtones. Can't find it anywhere.

Mad Scientist, this post is from 2007 so I doubt the OP is still reading. What you saw was perhaps bronze fennel or maybe Anthriscus sylvestris 'Ravenswing'. I do not believe there are any purple cultivars of dill. Btw several members of the Apiaceae family are poisonous so I wouldn't recommend tasting until you are absolutely certain of the id.