The colorbox image has an immediate background surround which is normally white in the examples. I emphasize that I am not talking about the dark background overlay. Although it seems an obvious requirement, there seems to be nothing in the settings that covers customizing this background color so how would I do that without diving into the plugin code?
Add this style to your stylesheet to override the plugin background color
#colorbox {
background-color: #FFF; /* Add a color of your choice */
}