Krystal Parrish shared a photo of the deep pink lights on her Instagram. She was heading from Chicago's Midway Airport to Washington D.C.
Twitter user Jennifer Moran also shared a photo from the inside of her airplane. It was unclear if all other Southwest flights lit up their cabins in pink.
A representative from Southwest told several media outlets they were unaware of details on the pink lights for that flight.
"Our flight crews celebrate, commemorate, acknowledge and share in special moments with our customers all the time," the statement said. "Some of our aircraft are equipped with mood lighting and while this was not a companywide initiative, at times, our flight crews will adjust the lighting for a customer or group of customers traveling on their flight."
Hundreds of thousands of men and women took part in the Women's March on Washington as well as in sister marches across the country. Celebrities such as Katy Perry, Zendaya, Scarlett Johansson, Alicia Keys, along with several politicians also took part in marches nationwide.
Many people wore pink cat-eared hats, which were created in response to leaked audio and video recordings of President Donald Trump making lewd comments about women in 2005 with Billy Bush.