Because of etymology: vert, verus. Thus, in English, verity, verisimilitude, and veridical, but also verdigris, verdure, and verdant. Thus, literary representations of truth as green, as in the allegorical four daughters of God―Justice, Truth, Mercy, and Peace―who first appear in Psalm 85. By the time they feature in the medieval play The Castle of Perseverance, they are chromatically clothed, with Truth sporting chartreuse.
Lauren Winner, “Alphabetic Art,” Image #98.