Anyone know how to take out the pattern of a shirt (i.e. remove a floral pattern) and keep the original shading/highlights so that you can now use the shirt to display a solid light color?

LL Bean does it with their turtlenecks, but they're solid colors. I'm having a hard time removing the pattern, mostly.

Here's the psd of one that's already been done, but I don't know how it was done...

http://www.mammasmilk.com/uploads/files/fabric_example.psd