Oddly enough, the MB emerger used around these parts is a Pheasant Tail Nymph tied as a soft hackle... which isn't really a light brown color... but it doesn't matter. I've tried a number of different styles and colored nymph patterns during the MB hatches and always end up doing better with a fly meant to represent the dun.
However, I've never tried a subsurface MB pattern with a red head so maybe be that's the key
However, I've never tried a subsurface MB pattern with a red head so maybe be that's the key