John Gill, on Prov 23:31

Look not thou upon the wine when it is red,.... Or, "because it is red" (r); or shows itself red. Which was the only wine used in the land of Canaan, or, however, the most esteemed of, and that most of art which had the best colour; when it had a good, bright, red colour, or sparkled, and looked bright and beautiful, so the word signifies; and then it should not be looked upon: not that it is unlawful to look upon the colour of wine, and thereby judge of its goodness; but it should not be looked upon with a greedy eye, so as vehemently to desire it, which will lead to an intemperate use of it; just as looking upon a woman, so as to lust after her, is forbidden, Mat 5:28; when it giveth his colour in the cup: or, its eye in the cup (s); such a bright, brisk, and beautiful colour, as is like a bright and sparkling eye. Here is a various reading; it is written in the text, "in the purse" (t); it is read in the margin, "in the cup"; and Jarchi's note takes in both, "he that drinks wine sets his eye on the cup; and the vintner sets his on his purse;'' when it moveth itself aright; sparkles in the glass, or goes down the throat pleasantly; or rather looks well to the eye, and appears right and good, and promises a great deal of satisfaction and delight. (r) "quia", some in Mercerus, Gejerus. (s) "oculum suum", Montanus, Gejerus, Michaelis. (t) "in crumena, vel marsupio", Mercerus.