"A green-white dress"
Translation:Ein grün-weißes Kleid
What exactly do you mean? Which behaviour precisely? Hyphenated adjectives aren't very common, but a rule is: the first word is used in nominative, while the second one gets inflected.
The compound grün-weiß constitutes a single adjective. The suffix -es is only appended once to the end of the adjective.
I entered "Ein grünweißes Kleid" and it was accepted, albeit it "corrected" me for leaving out the "-" and called it a typo. My German friend would type it without the "-" though, dunno if it's the spelling reform or what.
The hyphen is used to emphasize the border between coequal words like "green" and "white". So it is better to add the hyphen in this case.
The suffix -es of adjective weiß tells that the noun das Kleid is neuter.
- der weiße Tisch - ein weißer Tisch (male)
- die weiße Lampe - eine weiße Lampe (female)
- das weiße Kleid - ein weißes Kleid (neuter)
Does "grün-weißes" mean that the shirt is one color, a greenish white, or that it has both colors - for instance, that it is green-and-white striped?
"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".