Is it permissible for a tenant to make alterations and changes to the apartment she inhabits, such as painting, knowing that her financial situation is comfortable?

There is no harm in renovating or painting a rented apartment as long as it does not reach the point of extravagance, damage the apartment, or alter its structure, unless with the landlord's permission. It is good for the wealthy to minimize excesses in clothing, housing, food, and other matters, out of humility before Allah the Almighty.