I think after seeing and reading about all the stuff griffith wants people to understand the pain od the women and how she escaped the danger of soldiers fighting and shows the struddle of the mother during the civil war. Whereas, the other part where soldiers were fighting there was this red background depicting evil, violence or any sort of danger.
I think griffith wants people to understand that the blacks are equal to any other with and his cinematography urges the people seeing the film to think that Lynch and lydia are e to be supported and should be treated equal.