Cow dung has antiseptic properties and hence provides a literal threshold of protection for the home. It also provides contrast with the white powder.
Decoration was not the sole purpose of a Kolam. In olden days, kolams used to be drawn in coarse rice flour, so that the ants don't have to work so hard for a meal. The rice powder is said to invite birds and other small critters to eat it, thus inviting other beings into one's home and everyday life: a daily tribute to harmonious co-existence. It is a sign of invitation to welcome all into the home, not the least of whom is Goddess Lakshmi, the Goddess of prosperity. The patterns range between geometric and mathematical line drawings around a matrix of dots to free form art work and closed shapes. Folklore has evolved to mandate that the lines must be completed so as to symbolically prevent evil spirits from entering the inside of the shapes, and thus are they prevented from entering the inside of the home.
3x3 dot all and only symmetry 9
Goddesses Swastika Kolam with a single cycle by Nagata S, each
of which is corresponded to one of the nine Devi of the Hindu
or the nine Muses in Greek
Kolam Gallery - 1