Take white background out of image with cycles? How can you take the white background out an image texture in the cycles node editor?

Here's the image I found on the internet:



What I want to do is add scratches but have a different colour/texture underneath the scratches. Here's my current node setup:
To do that, you can use the grayscale map as the mix factor.

To mix colors:



Or shaders:



The color ramp node is only there to get rid of jpg compression artifacts.