Flavescent comes from Latin, stemming from the verb flāvēscere ("to turn yellow") and flāvēscēns, the present participle (turning yellow), the both derive from flāvus ("yellow"), meaning the word literally describes something that is becoming or turning yellow.  It's an adjective, first appearing in English around the 1850s, used for things like leaves in autumn or certain plants, indicating a yellowish hue.  

According to the Oxford English Dictionary (OED), the adjective flavescent was first used in English in the 1850s, with the earliest evidence found in the writings of botanist Asa Gray in 1853.  The verb "flavescate" exists, but it's largely obsolete, with its adjectival form, "flavescent," used to describe things becoming yellow, like autumn leaves or certain plant diseases (flavescence). 

Examples of "flavescent" used in a sentence: 
"During fall in New England, it's magical to walk through the park and admire the flavescent leaves flutter to the ground." 
"As I hiked through the forest, the path was covered in a thick coating of flavescent leaves." 
"The old parchment developed a flavescent tint during the time it spent on the bookshelf." 
"A flavescent mist hung over the marshlands, giving the dawn an eerie beauty." 
"Within minutes the miniature oasis was no more, a flavescent smudge of decay against the sickly, pallid earth." 
"Each fall, the leaves on the big tree in my yard turn a flavescent hue, in sharp contrast with the orange and reddish colors of the leaves in nearby trees."