The welding filler metal is a different alloy than the main tubes and would possibly anodize a different shade depending on the filler used to weld it. 7005 frames are generally welded with 5356 filler and the charts I've seen say it will match when anodized. 6061 frames are generally welded with 4043 filler and the weld would anodize darker than the rest of the frame. The chart says that a 6061 frame welded with 5356 filler would match, so you would have to know what filler is used on the frame you want to anodize.