Why does the tenant has to paint the property once the it vacates this?
This is a very subjective topic since it will vary from case to case. You do not have to pay for this if the condition of the paint is beyond the normal wear and tear considered for the time the tenant occupied the property.
What if they changed the color of the wall. This would probably be subjected to the tenants reaponsability correct?
You are correct!. Changing the color of a wall is not considered normal wear and tear. The tenant basically changed the physical aspect of the property to its taste. Therefore, they tenant should be responsible to paint the wall back to its original color or getting the cost of painting deducted from its security deposit.