More lights is generally better throughout the whole grow, true, but spectrum does have some effect. Bluer spectrum promotes shorter, more compact growth, while redder spectrum promote stretching & flower growth. You can grow beginning to end with a blue or red light, but it will promote different traits. UV also promotes more
terpenes, flavinoids & color in growth as well, as they're produced to help handle temperature or UV related stress (which is why colder temperatures during the night cycle also promote those traits), and why a full spectrum light is ideal in all stages (even if they do lean more blue or red)
