Krishna facial markings
Tilak, in general, is a mark of auspiciousness. Lord Krishna, the original Person, wears "Vishnu tilak," which signifies His divinity. In the Gaudiya Vaishnava tradition, the Deities of Krishna wear tilak that reflects the philosophical position of the worshipper.
The different Vaishnava sampradayas ("schools of thought") each have their own distinctive style of tilak which depicts the siddhanta ("philosophical conclusion") of their particular lineage. Some examples are:
In the Vallabha, Rudra-sampradayas, the tilak worn is generally a single vertical red line. This line represents Yamuna devi. The form of Krishna worshiped in the Vallabha line is Sri Nathji or Govardhana. The consort of the Govardhana hill is the river Yamuna. Their process of surrender goes through Yamuna devi.
The Madhva sampradayas mark two vertical lines representing Krishna's 'lotus feet'. In between is a vertical black line is made from the daily coal of the yajna-kunda (fire sacrifice). In their sampradayas, the process of worship involves 'nitya-homa', or daily fire sacrifices to Narayana or Krishna. The coal remaining from the puja (worship) is used each day to mark the forehead. Underneath the black line, a yellow or red dot is added to indicate either Lakshmi or Radharani. Those who did not perform daily fire sacrifice wear the simple two line tilak only.
In the Gaudiya Vaishnava sampradayas the tilak is usually made out of mud from Vrindavan. The main tilak is basically identical to the Madhva tilak. The slight difference arose due to the emphasis on the chanting (japa and kirtan) in the Gaudiya tradition. In Chaitanya Mahaprabhu's line, chanting is given as the essential devotional (bhakti) activity to be performed in Kali-yuga in preference to fire sacrifices. As such, the black line made from fire sacrifice ash isn't included.
The second difference arose due to Chaitanya Mahaprabhu's process of approaching Krishna. In the Gaudiya line devotees do not approach Radha and Krishna directly, but always indirectly through their servant. To indicate this, the red dot representing Radha is replaced with a Tulsi leaf offered at the base of Krishna's feet. In Gaudiya belief, only through the mercy of Tulsi Devi (or another pure devotee) can pure devotion to Radha & Krishna, or Krishna & Balarama be awakened.