I have to make calla lilies for a wedding in two weeks and the bride wants eggplant. I have made dark purple but not eggplant.
maybe add a little black to it.... Ive never done an eggplant color so maybe you will get some better suggestions.
To achieve eggplant/aubergine mix both red icing/fondant with royal blue or navy icing/fondant. If you are making BC try making it ahead of time one or two shades lighter than you need.. it gets darker with time
I use 3 parts purple, 1 part brown, 1 part red...that'll get you heading in the right direction but you may have to fiddle around and adjust the color proportions to get the right shade or depth of color...HTH