The Mercury Baetis is a great tailwater fly in the spring when the mayfly emergers are out. You can typically throw two of these in a tandem rig during a BWO hatch to imitate the baetis nymphs as they rise to the surface.
The most common color is BWO, however you can add variety by introducing Brown Z-lon or Rust-brown Superfine dubbing. You can also use a lighter colored thread if you are worried about the thread showing through when the dubbing is wet.
I did not have any Z-lon when tying this patter, so I attempted to tie it anyway with some left over materials. Needless to say - I needed the Z-lon to make this fly look good. The material I used was too thick and too stiff, and my legs didn’t turn out how I wanted them to.