White is not an uncommon color of mourning in traditional Indo-European folk societies. It was the color historically associated with mourning from Ireland (the banshee is associated with wearing white, always) to Eastern Europe, though not usually associated consistently with Christian Europe. It is also the traditional color of mourning in China and India. In the Catholic church, both black and white vestments have been used for funerary rituals--white being the color used for children's funerals.Weddings were also considered to be a time of mourning for the bride and the bride's family, as this little vignette about Russian folk weddings tells:
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Among the most fascinating are the items used in the ritual of "parting with virginal beauty". Here we see the bride's headpiece, a ribbon, a miniature pine-tree decorated with ribbons, and an artificial braid. These articles all symbolized childhood; hence, this ritual signified the fact that, on marrying a man, a girl has to part with her childhood and all that is associated with it. The ritual "parting with virginal beauty" was performed solemnly in front of the bride's parents, brothers, sisters, and friends.
Of all the wedding exhibits, the costumes attract the most attention. An exceptionally beautiful garment was worn for the showing of the bride, when the groom and his parents would come to view her. In complete contrast is the costume worn by the bride at the shower party, which involved a show of ritual mourning. The material for this costume was composed of traditionally mournful colors, such as blue and white, and the costume was completed by a long white towel or scarf that veiled the bride's face. The Russian peasantry believed that it was necessary for the bride to dress as though in mourning, as this symbolized the death of her present life, with which she must part in order to prepare for her rebirth into a new life after the consummation of the marriage. Also exhibited here are costumes worn for the wedding ceremony itself, as well as those worn for the wedding party. For the "morning after the wedding night" ritual, both bride and groom wore particularly bright and festive garments, with a predominance of red symbolizing the new life, youth, and beauty of the marriage."
You can view the beautiful dress and read all about it here:
http://www.ethnomuseum.ru/---English---/love.html
Does that mean that "She'll Be Comin' Round the Mountain" is about mourning? I dunno. Considering the song is, as I understand it, based upon the African American spiritual/work song "When the Chariot Comes" I have no clue what significance the color white has. The cultural meaning of ritual colors is extemely complex, and I'm not at all familiar with the color associations on the continent of Africa, though I'm sure the associations are varied, as they are elsewhere.