In later versions, when you punch a sheep to kill her, all the entity got red-colored, in order to let you know that the sheep has been damaged. Now, the wool of the sheep still white, or brown or any color, and the legs and head are red while punching the sheep.
Linked issues
duplicates
Attachments
Comments
Confirmed, using Windows 8.1 amd64 with 64-bit Java 7 Update 51
I can confirm. In flatworld slimes do not become red when hurt.
Using Linux, x64.