I swear by the twilight of the evening.
I solemnly swear by the rosy dusk
I swear by the twilight
But nay, I call to witness the sunset redness
So I do not swear/make oath with the twilight
I (God) swear by the impressive red color of the dusk
So by oath of the late evening’s light
But nay ! I call to witness the glow of sunset
So I swear by the evening glow (i.e., the twilight or its later glare)
Behold! I call to witness the twilight of sunset
So I swear by the afterglow of sunset