To my knowledge there has been no conclusive evidence that sun/no sun darkens or lightens leopard torts. One book may have mentioned this as speculation. It is true that a light colored animal may add darkness at a greater rate than light and vice versa but that is its makeup just like star species add color at varying times throughout their life or why 2 identical siblings grow at different rates given the exact same husbandry.
Bottom line- leos have color variations and do not all end up identical as adults. A safe bet would say the darker one will always be darker as an adult and vice versa.