The tilak invokes a feeling of sanctity in the wearer and the others. It is recognized as a religious mark. Its form and colour vary according to one's caste, religious sect or the form of te Lord worshipped.
In earlier times, the four castes (based on varna or colour) - Brahmana, Kshatriya, Vaishya and Sudra - applied marks differently. The brahmin applied a white chandan mark signifying purity, as his profession was of a priestly or academic nature. The kshatriya applied a red kumkum mark signifying valour as he belonged to warrior races. The vaishya wore a yellow kesar or turmeric mark signifying prosperity as he was a businessman or trader devoted to creation of wealth. The sudra applied a black bhasma or kasturi mark signifying service as he supported the work of other three divisions.
Also Vishnu worshippers a chandan tilak of the shape of "U", Shiva worshippers a tripundra of bhasma, Devi worshippers a red dot of kumkum.
The tilak covers the spot between the eyebrows, which is the seat of memory and thinking. It is known as the Aajna Chakra in the language of Yoga. The tilak is applied with the prayer - "May I remember the Lord. May this pious feeling prevade all my activities. May I be righteous in my deeds." Even when we temporarily forget this prayerful attitude, the mark on another person reminds us of our resolve. The tilak is thus a blessing of the Lord and a protection against wrong tendencies and forces.
The entire body emanates energy in the form of electromagnetic waves - the forehead and the subtle spot between the eyebrows especially so. That is why worry generates heat and causes a headache. The tilak cools the forehead, protects us and prevents energy loss.