A wig fades. The sun takes tone and shine from the hair, and there is nothing to restore them from within: no scalp underneath. So a wig is usually coloured not to change the look but to bring back the colour it had. The shade is chosen from what is left in the hair and from its condition, not from a catalogue.
Strong lightening is a separate matter. It leaves the hair dry, brittle and less elastic, and in a wig that damage stays for good: on a head the damaged hair is replaced by new growth, here nothing replaces it. A very light wig is therefore better made from light hair in the first place than bleached once finished.
