I'm trying to edit the Greybird gtk/xfce theme so all blues are replaced by greens which has been easy using the gtk theme preferences utility. Problem is all the little .png files for the buttons etc in the metacity & unity folders are in blue

I need to find an easy way to change all of these little png files. Say we have a blue colour #398EE7 it must be changed to #39E78E so in essence I'm swapping the green & blue byte values around. Every png has several shades of blue in it to create a gradient for the little buttons so quite a few values need change.

Any ideas?

Please be specific in your responses as I know zip about gfx art and don't have a single artistic bone in my body

Screenshot sample of a blown up image in gimp,