Not sure the labia are darker. In any case, I find the natural differences between girls in lip size and color to be far more pronounced than any before-after changes that might result from giving birth.
Then there are the racial differences - with the Asian pussy being the finer animal. (I have a little shrine set up in my living room where I always keep incense burning before a Song dynasty jade carving of the Chinese Pussy God. May she continue to smile on me.)
Also, a pussy right after birth will be a lot looser than a few mths or years after giving birth, tho they never seem to revert to their former tightness.
But it's all good! I myself am an equal-opportunity DATY artiste.