"He was a thin leathery man with colorless eyes, so colorless that light did not reflect his eyes". What kind of construction is that? I usually see similar sentences being split into two sentences.
As it is a sentence from Harper Lee's magnificent novel To Kill a Mockingbird, the construction would be termed brilliant or something like that.
Coming from the pen of a mere mortal, the sentence would be called stringy or strung together.
What makes this sentence work so well is the appositive clause to the first colorless that at once overstates the adjective and artfully moves the word "eyes" and avoids a possibly anticlimactic prepositional phrase at the end.
(The implied phrasing is "He was a thin leathery man with colorless eyes, [eyes] so colorless that light did not reflect [in them]."
(This is marvelous literary work, but as the Latin proverb says, quot licet iovi, non licet bovi - What Jove may do is forbidden to the ox. We the lowly might be marked amiss for similar things.)