The Great Tit in the above photo is a bird hatched this summer.Note the contrast between the greater coverts and primary coverts,the primary are a paler bluish/greenish/grey.When the bird has its full moult next year in either ( June,July or August) it will then be in adult plumage and there will be no contrast,Blue Tits can also be aged in this way.
( in the photo its looks just a paler greyish/blue)