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.
That's it!
هذا هو
Der war es!
¡Ah! ¡Eso es!
Hai ragionissima!
Isso aí!