A greywing budgie has an even shade of up to 50% diluted body color (depending on body colour - blue base or green base). So the body colour is around half lighter than normal.
Feather wings are light/dark grey (instead of black like Normals have) with yellow edges for green birds and white edges for blue birds.
Throat spots are grey. Cheek patches are light violet. A tail is grey with a bluish tinge instead of dark blue in the Normals. There is also a variation of greywing called: fullbody greywing, where body colour is almost undiluted and cheek patches are dark. Wing feathers are grey though.