The WD58 glass should be color neutral for normal light sources.

Auto white balance is based on the overall image content. When we attached a wide angle the field of view is much greater, and perhaps the color balance of the wider image is different than the narrower image due to different colors at the edges of the wider image. This can fool the auto white balance to giving a different setting.

Try usng manual WB, and zoom in to the same field of view when setting white balance, then zoom out.

Over three years later .. Richard said:
 

I recently purchased a wide angle adaptor for my Sony Handicam .. and liked it so much that I decided to get the Cannon adaptor for my GL2. Best news is that B&H Photo had a used one including shade (rated +9) so I snapped it up and saved around $50.00! :-))

Just received it today and have only mounted it on the camera. BOY is it impressive looking. I'm glad to have it in my "arsenal" of accessories for my GL2

Back in the "old days" I remember purchasing inexpensive lenses for my 35mm still camera (inexpensive = not the same brand name as the camera).

I *do* remember that there was a distinct color shift over the stock lens. It was only when I shot slides since the printing process of negatives would often compensate for slight shifts.

I'm not saying that this is the case now, but it is possible to have a color shift even though you can't notice anything when looking thru the lens with your bare eye.