This depends on the specific spell in mind. But in concerns of illusion spells that are legit, they will work. But they are only temporary. Illusion spells work in that they alter the perspective of what is seen. For example: with a eye colour spell, your eyes won't actually change colour from the spell. What happens is that the illusion is made and this illusion acts in a way that alters what people see. Your eyes will still be their natural colour, but will be seen as the intended colour - if properly casted and works.