One way to tell the difference is the presence or lack of papillae, those little raised things some lilies have on the inner side of the tepals. Nymph has them; not Bonbini. Nymph tends to be lighter inside around the base of the tepals where the papillae are located. Most photos won't show this but if the angle is right the papillae can be seen. Pirl from DG Plantfiles has a good photo of Nymph showing the papillae; it is kind of a side on shot between the tepals, you can see them sticking up.
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