When you get excited both your inner and outer lips change:
They increase in size and can become two to three times their normal scope, comparable to a penis that gets erect.
Because the inner lips increase in volume they separate the outer lips, which gives the penis easy access. This is further facilitated by the natural lubricant that leaks out of the vagina and spreads to the inner lips when you get excited.
The inner lips change colour. They can turn pink, bright red or even purple. When your excitement disappears, the colour will return to normal rather quickly.
After an orgasm the lips quickly return to their normal size. But when you don’t come they can stay swollen for quite some time.