Oh yes - I see what you mean,
Yes - that color is in the configurable.css (line 157), but if you remove that color it would remove it from behind the 'Add to Cart' area too.
The easiest - and probably most effective way would be to edit the .productDetailWrapper .image .imageWrapper by adding a white background color:
webassist/plugins/catalog/css/products_detail.css (at about line 10)
.productDetailWrapper .image .imageWrapper {
background-color: #FFFFFF;
}