Even with "apply colors to non-Qt applications" option set, GTK2/GTK3 applications are not affected by KDE color configuration. Seems like kcm_colors places colors for GTK2 to .config/gtkrc and .config/gtkrc-2.0, while kcm_gtk places GTK2 configuration to .gtkrc-2.0 and .gtkrc-2.0-kde4. Probably, kcm_gtk and kcm_colors should use the same configuration files? I couldn't find where colors for GTK3 are placed. Could it be that they are not set at all?
*** This bug has been marked as a duplicate of bug 353071 ***