Why "mhath" not "math"? Because "briogais" is feminine?
No. Although briogais indeed is feminine, here (as indicated by the comma) the glè mhath does not attribute briogais, it’s general very good! exclamation.
Lenition here is caused by glè very which always (if possible) lenites the following word.
Why is gorm for green wrong here?
Gorm means blue. For some reason I've yet to understand, Gaelic considers grass to be gorm rather than uaine.
Along the same lines, dearg means red and glas means grey, except when it comes to hair, when ruadh means red and liath means grey.