A second mount in the same or a complementary colour tends to give more interest depth to your image. The effect is achieved by exposing a small amount, perhaps just 5mm, of inner mount between the window of the outer mount and edge of the image. As a general rule, the outer mount is a lighter colour than the inner one.
Strong and darker colours, which would be overpowering on their own, work well for the inner mount. A light colour inside a dark colour can also work very well when an image is dark. In a case such as this,10 -15mm of inner mount should be revealed.