Saraswathi Devi and Lakshmi Devi are shown sitting on a Lotus flower.The color of Lotus is white for Saraswathi Devi and red for Lakshmi Devi.
There is a science behind depiction of Saraswathi Devi and Lakshmi Devi sitting on a Lotus.This is because the first movement involved in creation is in the shape of a lotus. Even our thought waves travel in the shape of a Lotus.This is similar to putting a stone in a pond.The waves traverses in all directions then come back to the point the stone was dropped and if we draw lines connecting the waves to the point the stone was dropped, it develops into a beautiful Lotus.
The white colored Lotus symbolizes purity and knowledge as Saraswathi devi is the embodiment of pure knowledge.The red colored Lotus symbolizes wealth and prosperity.This is the reason behind Saraswathi Devi and Lakshmi devi being portrayed as sitting on White and Red Lotus flower respectively.
The Symbolism of Lotus in Hinduism taken from the speech of Jagadguru Swami Sathyananda Saraswathi Tiruvadikal. Listen to the concept in Swamiji's own words below. http://youtu.be/dItAI4JnUGQ