Colored shadows are the result of different colored light sources in different positions. There might have been a light source you missed, or maybe there was something that was having a prismatic (light splitting) effect on the white light source that you saw. Possibly the fence was creating some light scattering that caused this effect.

http://www.learner.org/teacherslab/science/light/color/shadows/coloredshadowsbackground.html
http://www.exploratorium.edu/snacks/colored_shadows/index.html
http://blog.sciencegeekgirl.com/2009/05/27/physics-toys-tuesday-colored-shadows/

~ Becky in Tucson