Sigmas do usually have a yellow or tobacco stain cast, the 18-50 does it too. It's like a warming filter, or taking daylight shots with the shade white balance. So what are you doing comparing it under yellow tungsten light, trying to equalise the color with the white balance dropper and messing with the tint as well? Your experimenting might have been more valuable if you had provided a general correction value for this cast. And as for anyone who doubts the Sigmas have this cast, it's laughably simple to see: you only have to compare it to other lens brands on the thumbnails on anything that is white. To a photographer that is used to post-production, they can see it on landscapes and other types of shot as well.