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.
You can check out the original comic here! http://info.nicovideo.jp/seiga/book_anime/#m1_gyaruko
Official Galko-chan Nico Nico Page and Episode List: http://ch.nicovideo.jp/channel/ch2617618
Episode 2 - "Is It True That You're a Rich Girl?" : watch/1453955050
Episode 4 - "Is It True That You Can't Help Moaning?" : watch/1455085270