In Flexibility 2, you can make your background any color but you'll still have the top gradient fade in the background. FlexSqueeze allows you to remove the gradient fade but not Flex2.  As far as a header designer goes, you might go over to Warrior Forum and post a request.  There are several good header designers there who could probably help you out.

The grey color in the nav bar is the background image used for your navigation.  It's a transparent PNG image, so whatever color you use for your navigation, the color shows through the image. In order to get rid of this image, you will either need to edit your style.php file to remove the image, which can be a little bit hairy because there are a lot of php if/else statements for the navigation bar and if you mess one up, your theme will not work.  

The other solution would be to create a small completely transparent PNG file and overwrite the existing navigation images.  On your local computer, look in the images directory for the theme and you'll see several navigation images, both a left and a right image for each style of navigation tab available. It uses the CSS sliding doors technique for the button hover effect.

If you don't plan on using any of the navigation tabs, you can simply overwrite all the navigation images so you don't have to figure out exactly which ones your theme is using. Name the files identical to the one's in the images directory and upload them to your server. You will then have no background image show in your navigation.

Sorry about the long work-around.