"A green-white dress"
Translation:Ein grün-weißes Kleid
"Grün-weiß" means that both of the colors are present. But note that for neighboring hues like "blaugrün" a color somewhere in between blue and green is meant. Similarly, "rotbraun" means a reddish brown.
EDIT: In the past there was a rule that mixed colors (greenish white) were written as one word grünweiß while combinations of distinct colors (green and white) were written with a hyphen grün-weiß. This rule was very handy and I don't know exactly why it isn't mandatory anymore. Nonetheless, you can still use it, that's what I do and that's what I recommend. If you want to make it absolutely unambiguous that you are talking about a greenish white, just say "grünlich weiß".