Responsa for Menachot 79:5

ולידרשא בפירקא גזירה משום קלא אילן

Then let it be announced at the public lecture! - It is to be feared that people will use imitation blue.<span class="x" onmousemove="('comment',' , a vegetable blue dye, probably indigo, being an imitation of the genuine blue which is obtained from the blood of a mollusc. Now the prohibition of diverse kinds is waived only when woollen threads dyed with genuine blue are used, but not when they are dyed with imitation blue.');">⁸</span> But it is no worse than if it were white!<span class="x" onmousemove="('comment',' The threads dyed with this imitation blue should be regarded as though not dyed at all, and it has been established that in the absence of blue threads ordinary white woollen threads may be used in their stead.');">⁹</span>

Teshuvot Maharam