According to Vastu shastra, the element of south direction is Agni i.e. fire and the sign of fire is red color. Hence south direction is related to the red color. This direction is also related to the summer season. Both fire and red are the most prosperous during summer season.
By getting the red color painted in the south direction or keeping red things in this direction, good results related to the fire element in this direction are obtained. Since this direction is also related to the middle girl child of the house. Therefore, by getting the red color in the south direction, as well as keeping things related to fire in this direction, the middle daughter of the house will get big benefits.
This direction is also related to the eyes. Therefore, if you have any problem related to the eyes or have to get your eyes checked again and again, then you make amendments in the south direction according to Vastu shastra.