True purple strains are short and squat with big fan leaves and tends to produce smaller but very dense buds. I see you have3 707 in your screen name, you should check out some of the clubs in the area. Blue Sky has a bunch of different strains and if you want a purple that has a good yeald the check out Querkle from Blue Sky.
From Blue Skys Oaksterdam Nursey Fall 2008 Catalogue
Querkle
Genetics: Purple Urkle X Space Queen. Flowering time: 8 weeks
Effect: cerebral
Bounquet: Grape taste & fruity smell
Grow Environment: Indoor
Plant Height: Short and dence
Company: Subcool/TGA-Found at Blue Sky Coffie Shop in Oakland CA
Fans of purple strains will want to check out this mostly Indica strain. It is known for it's grape taste and sweet, fruity smell. Querkle is a short and dencs plant, with a good yield of purple/orange buds.
Hope this helps. hey what part of the 707 are you?