negative text
 

Would like to see this done
1. Photo
2.Text on top of it
3.all pixels of text to be negative of underlying photo.

Can't find any samples on google, I guess I can't think of appropriate keywords.

Doing image wattermark, having semitransparent white turns bad mostly.
So the upper idea might work.

Any samples on net where this is done - text negative of photo ?

using masks is described here:
http://www.imagemagick.org/Usage/masking/
now you make your text and use it as a mask (just like in photoshop)

or crop it using mask made with your text, make negative of it and apply it on original image.
you make negative like that, if memory serves:
convert input.png -negate output.png