Ohh, sorry. We read that you want to change existing grey color. However, if you want to change just description content, you can do this under Appearance > Menus section. Under each item you have description field. But if you don't see it, please activate it under Screen Options section in the right top corner.
Comments
description color can be changed with as extra css only:
#Top_bar .menu > li > a span.description { color: #000 !important; }