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.
Inverted nipples are when the nipples 
retract back toward the breast.
تكون حيث تتراجع الحلمات نحو الأثداء
Das sind Nippel,
die sich in die Brust zusammenziehen.
Se tratan de los pezones que
 quedan hacia dentro de la areola.
Hein ?
Si dice che sono introflessi quando 
sono ritratti all'interno del seno.
Mamilos invertidos são aqueles 
que se retraem de volta ao peito.