Galko's skin got really dark as a result of tanning.
Otako comments that since Galko's skin has tanned so much, her nipples must be dark as well. Along with Ojou, Otako yells that Galko has dark nipples.
R-Right.
حـ... حسناً
J-Ja …
Entendido.
S-Sì.
Ah, tá...