Al-mufattal derives from the root f-t-l. Form II fattala means to twist or plait, and mufattal is a passive participle, “twisted” or “plaited.” Its mufaʿʿal pattern reveals the derivation.
Here al-mufattal qualifies al-dimaqs and appears in the genitive. The twisted texture makes the simile about more than color: the visual structure of the material matters.
By contrast, mufattil would be an active participle, “one who twists,” whereas mufattal names something that has undergone the twisting.