Yes, it is.
First, she already knew a little something was gonna "leak", anyone who has nipples knows that when a breeze hits you can tell which one is uncovered and which isn't, and a woman even more so since they're even more sensitive. Naughty girl.
Second, that's not a pasty
the tape mark would be visible
you can clearly see it's skin + nipple
And third, not every woman will have dark nipples after becoming a mother like some claim, so yes, that's her natural color.