this is gonna get a lot of shit even tho is a good coloring, some people dont seems to realize the cover was draw with the intention of been colored, they rest wasnt there is not way the quality will ever match because the doujin was meant to be printed on B/W, to make it match the quality of the cover would mean redraw EVERTHING.
yes some areas lack shadows or reflaction in the skin like in the cover, because they werent on the B/W print, and unless you can mindread the author to know how did he intended to draw it if he would draw it on color all you have to work with is the original B/W.
my only recomendation to the @mooncake7 if you recolor it your self try using a slingly darker skin tone. she looks to pale, you might want to consider copy color of the skin on the cover. there you can see her skin on bright light like in the face and the tits. and the darker tone on the shadows on ther underboob and the crotch.