Go to /public_html/templates/gk_yourshop/css and open templates.css. Then find this line:
#gk-mainnav > div.gk-menu > ul > li > a { color: #383838; font-size: 11px; font-weight:bold; text-transform: uppercase;height:43px; line-height:43px; display:block; }
Change the #383838 (if that is what it presently is) to the colour code you want.
You may want to also change the roll over colour aswell. If so, find the following line (which is only a few lines further down within the same css file:
#gk-mainnav > div.gk-menu > ul > li:hover > a { background: transparent url('../images/style1/menu_bg.png') no-repeat -459px top; color: #fff; left: -1px; position: relative; }
Simply change the #fff (or whatever yours presently is) to the colour code of your choice.
Enjoy