if you have a good idea of what you want, make sure you explain it clearly to the designer you choose. if there are colors you want used, send samples to him/her. if you want one of the color specifically for the background, let your designer know which one and what you want it used for.

if you really don't have any idea what you want, either figure out what you want in advance so you can express it clearly or find a designer with at least 3 designs you REALLY like - designs where if your site had those layouts and colors, you'd be happy - and tell the designer which designs you like and what you like about them.