How can I get more colors in emacs vc-diff?

To just modify the colors of the added and removed lines:

(custom-set-faces
 '(diff-added ((t (:foreground "Green"))) 'now)
 '(diff-removed ((t (:foreground "Red"))) 'now)
 )

See Treys answer for a more complete customization.

The package diff-mode-.el provides more colors.

You can customize the colors by either customizing the faces diff-indicator-removed and diff-indicator-added, or changing the variables diff-indicator-removed-face and diff-indicator-added-face to be faces whose colors/properties you like. There are also changed varieties of the face and variable.

Other packages for enhancing diff-mode can be found here.