Nidvas (Donations) Hamishkan any silk. The reason for it is, for
building of the Mishkan we may not use materials that come
from unkosher animals. (silkworms are non kosher)
תולעת שני (red wool) The dye does not come from a worm.
It comes from a red seed that has a worm in it.
(Rambam Parah Adumah 3:2 – Rashi Yeshayahu1:18)