Steve,
somehow I find it difficult to believe, that this colorcast is
produced by the Sigma lense.
Here is your Sigma Image again, this time with a quick and dirty
color correction applied to it:
http://idisk.mac.com/ghrdt/Public/29419251-M.jpg
Looks pretty much like your canon-pic to me:
http://missionphoto.smugmug.com/photos/29419252-M.jpg
Of course you can yellow-balance a Canon image and white-balance a Sigma image and they look closer, though your Sigma image still looks quite a bit yellower. You need to yellow-shift the Canon image a little more to get a better match. But this will still produce unwanted casts across the color range in other areas. Even the neutral Canon lens will look odd if you intentionally yellow it up, we've all used SPP's color wheel.
For another properly reference, here are the two images again but using the cow in the background as a WB reference, instead of using the same preset color temperture. I plucked the cow right between the eyes in SPP, separately for each picture. I lined up the dropper in exactly the same spot in each trial, but being a little off left/right/up/down made no significant difference:
I agree with Lin that, in both cases, SPP shifts overall WB too far yellow using the dropper. This problem doesn't help people given Sigma's lens casts. Although in this case the Sun was low and the scene looked reasonably close to either Canon balance. Regardless, the relative cast between lenses remains blatant:
The yellow cast in your pic is simply a WB-issue, that you -if you
will excuse me- somehow produce to advertise your
conversion-business IMO.
I don't have an conversion business, unless you consider happily losing a lot of money so that myself, others, and even you may benefit from a greater selection of usually better glass, a business.
Maybe the Sigma lenses are slightly more yellow than the canons,
but most definately not that much.
You have to see the viewfinder to believe it, I suppose, though obviously SPP embeds all settings in the EXIF so its easy to verify that I'm showing you the simple truth. Why not borrow a cheap Canon SLR and look through both viewfinders using some of the lenses I've rated for you, above.
From that point on you'll want to convert glass for yourself too--using knowledge that I was kind enough to pay for, do the work, and give to you.
