Shiro Gal to Kuro Gal no Yuuwaku

Rei, the white gal, and Natsuyu, the black gal, gets their classmate "Otaku-kun" involved in one thing or another. You'd think the two of them were seductively tempting Otaku-kun, but the one who was interested in Rei was the black gal next to her, Natsuyu...

(Source: Kadokawa, translated)