I have more BWO imitations in my box than any other fly in my MF dry box. As far as color, I tie a light version (almost yellow with an olive cast), a med olive and a dark olive, some in parachute, catskill, and comparadun style. I tie them from size 14-24. The Cornuta is a big BWO (#14) and when you flip it over, it's actually a tan color like a March Brown. Have some for the D or Catskill streams in May. Many of the smaller olives hatching later in the season are darker in color. Also, for fussy fish, I tie a klinkhammer type with an olive quill body and an dun antron, or trimmed bright foam post for visibility.