The colour shifts occur during printing process of the stamps. These errors and flaws vary a lot. 
The ones shown here have made a difference. The colour shift changes the appearance of the original picture.  For Example: 
Yellow flowers have green colour on it, Horses have multiple ears and Reins. On other stamps you see: Split Twigs, Lipstick out of place, Collar shift etc. All these shifts and variations are explained in our online shop with images.