The Odd Sheep is the sheep in the middle. When viewed through a prism, the odd sheep turns blue and yellow, while all the sheep around it turn cyan and red as shown in the image below.

To view the image. Hold your prism with the largest side close to the screen with the title displayed in the bottom. Slowly pull the prism away from the screen while looking at the title through the bottom pane of the prism you will see it gradually change from 3 colours to black and white. When it turns black and white you are at the correct viewing distance.

Look at the entire image and you should see this. The Center sheep turns blue and yellow while the rest of the sheep turn cyan and red.