"I want a small white star."
Translation:Dw i eisiau seren fach wen.
In "Modern Welsh: Comprehensive Grammar", Gareth King says:
"In adjective sequences of the type 'big red bus' or 'dear little creatures', Welsh is more consistent than English, preferring always to place the size term closest to the noun: bws mawr coch, creaduriaid bach annwyl."
gwyn is one of the handful of adjectives that has a separate masculine and feminine form: masculine gwyn, feminine gwen.
And since adjectives after a feminine noun undergo soft mutation, gwen changes to wen after the feminine noun seren; thus, seren wen.
Have a look again at the Tips and Notes for the Colours lesson: https://www.duolingo.com/skill/cy/Colours