Like with framing images also with using mounts on them it is all about underlining the aestethic potential of the image. This not only includes the decision whether to use a passe partout but in case you decided to do both size and colour of the passe partout.
And the general rule with this again is:
The colour of the mount ought to correspond with the image. A plain white mount is not always the perfect choice.
The size of the mount ought to be chosen in a way that the mount will focus the viewers attention on the central motive of the image.
Have a look at the following examples to see the difference between the same image being framed using a mount and being framed without and the effect of the mount colour.
