It is a tradition, for some patterns, particularly in non-classic wet flies, to use red thread for the pattern regardless of other components. It is, of course, totally unnecessary. The above pattern could have been tied with tan, brown, black, grey or ? thread. Now, here is another option I have used (not in awhile) and seen others use, particularly if they are not given to using bead heads: they use red thread to designate weighted flies. The above fly could have lead wire wraps beneath the upper abdomen and thorax dubbing. It is concealed and short of jiggling the pattern in your palm, a good identifier would be use red or a similar color to identify your weighted patterns. Even a bead head can pack additional weight in behind the bead and beneath the body material.