Don't understand the fish's fascination with purple flies (any icthyologists to explain a trout's visual acuity/spectral sensitivity?), but it seems to work. Here's one take on Andy Carlson's pattern.
hook - Dai Riki 320 #16
thread - Uni 8/0 purple
Tail - hackle fibers cree
body - biot purple
wing - Congo Hair white
thorax - dubbing purple
note - using a #16 here because the only purple biots I had were from a duck; shorter than goose or turkey so I went with the smaller hook. Depending on what body material is used, I've tied these up to #12's; consider this a flatwater edition.
Part 1
Mash down barb, start thread at 70% mark and create thread bump
pull off a bunch of hackle fibers, even tips and measure; lay the fibers on top of the hook to mark for length (for duns, I usually use shank length)
refer to Rusty Spinner SBS for tail tie-in instructions (note I clipped the fibers off at the wing tie-in spot this time; thanks Steven for the reminder)
tie in moistened biot notch side down for segmented body (notch up if you want it smooth)
wrap biot forward to 70% spot and tie off
tie in Congo Hair on top of hook shank (2 wraps is fine)
grab Congo Hair, pull up, create a post with thread wraps; brush post with a little Sally
prep hackle (upsize hackle by one - for a #16 fly, use a #14 hackle); grizzly or grizzly/brown would be fine, too. I just happened to have an older Charlie Collins cree with a few feathers this size
tie hackle to post with shiny side out