TO INDENTIFY MISTAKE

Hi all,
selected the image of staircase shot. it was shot at f8. Wanted to blur the background at 2 levels of DOF.
1) Selected area by polygonal lasso L + quick selection W. Also refined the edges through the same. 2 separate layers for 2 locations.
2) Saved selection in alpha channel.
3) Blurred though smart layer, masked the selection, Field blur.
4) Getting the green cast in blur adjoining to railings. was getting the same issue through Shape and Gaussian blur instead of Field.
5) for this post, enhanced the blur amounts a bit.

unable to understand why the cast and mistake in my workflow process.
any of your pointer shall help me in correcting.
Thanks.