Hello,
Both of them are actually different shades of gray. The background is going to be based on the module class suffix you have assigned to the module. For example by default the module suffix is:
Template.CSS - Line 571
.module_round_box-gray_large, .module_round_box-gray_hover {
background: none repeat scroll 0 0 #f6f6f6;
color: #8e8e8e;
padding: 44px 48px 48px;
}