Is it a Genetic Mutation?

We have had some calm weather over the last week until this morning when the rain and wind came. One of the birds I caught before today over the previous sessions held in my garden was this Blue Tit below.

This picture shows discolouration of feathers on the wings, including the Primary and Secondary feathers and discolouration of the tail feathers as well

On the wing you can see the last 2 Greater Coverts, all the Primary Coverts and the Alula feathers, which are all coloured this peculiar brown. Presumably the last 2 Greater Coverts, Primary Coverts and Alula feathers are all old and could be from the bird being born last year, which would age it as a 5. I did however consider ageing it as a 4 but concluded since all feathers had a tinge of brown (the new green) except for the replaced Greater Coverts which were blue it was indeed a 5, as you would expect to see from a juvenile having moulted these feathers and in this case retained the 2 brown ones. If it was a bird from last year, presumably the replaced Greater Coverts would have originally been brown as well? In addition the Primaries, Secondaries and Tail Feathers tinged brown are probably the same age as the Coverts and Alula.

I do not know what causes the discolouration. Could it be a genetic mutation? I thought it could have been at first but then I thought if it was, then the Greater Coverts that have been replaced would still retain the brown colouring. Therefore could the change in colour pigment be associated with a past health issue or diet issue of the Tit. If anyone has any knowledge of this I would love to hear about it.

The birds health seemed fine too me, feisty as all Blue tits are and flew away with no problems after being processed and photographed.