"Mae hi'n wyrdd."
Translation:It is green.
No, soft mutation turns "gwerdd" into "werdd" :)
It's one of the adjectives that has separate feminine and masculine forms, so if it's right after a feminine noun, you use the feminine form gwerdd, lenited into werdd.
Modern Welsh: a Comprehensive Grammar says:
Note: even these few adjectives that do have a feminine form often revert to the masculine when separated from the noun by other words, particularly yn. Compare:
Stori °fer a short story
but: Roedd y stori’n °fyr iawn the story was very short
All the same, °fer would be acceptable here as well.
("°" indicates lenition)
So this seems to be what happened here -- after the yn, the default/masculine form gwyrdd was used even though the subject is hi.
And the lenition occurs due to yn before a noun or adjective, rather than because the subject is feminine -- I believe that "he is green" would be Mae o'n wyrdd/Mae e'n wyrdd, also with lenition.