"A white suit"
This is explained in the notes for the section 'Colours'.
A few adjectives have specific feminine and/or plural forms, and this includes some colours. Here, gwyn (white) has a feminine form gwen which is often used, although not everybody does so nowadays.
Adjectives take a soft mutation after feminine nouns. Siwt is a feminine noun, so you can use either siwt wen if you want to use the feminine adjective, or otherwise siwt wyn.