I am plotting a contour as 

    ContourPlot[f[x,y]==f0,{x,0,1},{y,0,1}]

  

and I want to use my custom `ColorFunction->Function[{x,y}, ... ]`  to color the contour (so that the contour varies its color).   So far, I have not succeeded and not found an answer on forums.  
Any ideas? 
Thanks,
Sergey