What is the purpose of wearing black on the day of a funeral?

black is known as the color of Mourning and Grief , which is suppose to symbolize the pain that is being felt due to the loss of their loved one.; The custom of wearing unadorned black clothing for mourning dates back at least to the Roman Empire, when the Toga pulla made of dark-colored wool was worn during periods of mourning.

You should also be aware that while black is the color of morning in western culture, (apart form medieval European queens who used white rather than black) other cultures adopt different colors for this (e.g. In Egypt yellow stands for mourning while in Tialand purple is the color of mourning)