taken from the PWCCA website:

"The photo on the left depicts a champion black and tan Pembroke Welsh Corgi male who has white markings on the top of his muzzle, chest, around his neck as a narrow collar, on his legs and underparts. This is commonly referred to as a tricolor. The black encompasses his eyes which makes him what is called a black headed tricolor. The photo on the right is of another champion male that shows the typical markings found on a black headed tricolor. Please note that it is perfectly acceptable to have a Pembroke Welsh Corgi that is black and tan with no white, but a Pembroke Welsh Corgi that is black and white with no tan present is a mismark. The tan is often not present at birth, but appears within a few weeks after birth."