The ending of the adjective changes with the case of the noun. In this case "water" = "das Wasser", and the adjective ending with "das" is "-es". If it were masculine ("Der tee"), it would end in "-er". If is feminine (die Frau), it would end in "-e". Here is also a discussion on it where someone else asked a similar question: http://duolingo.com/#/comment/10061?from_skill=20a5dd4deb36bfa484843b075d1d9780
only one problem das ending is e and a nuter indefinite ein is es https://www.duolingo.com/skill/de/Colors/tips-and-notes
For other people who like me, were confused by the tips and notes, they only present two ways of declining a noun:
- After a der Word, aka. Weak inflection.
- After an ein Word, aka Mixed inflection.
The third way is Strong inflection and is used when (amongst other things) there's no article. This inflection has slightly different endings, including the -es ending for neuter nouns in the nominative which explains rotes Wasser.
The Wiki page is actually quite helpful for this.
der Wein is in the nominative case because it is the subject.
rotes Wasser is in the nominative case because it is a predicate that says something about the subject -- such predicates are (almost always) in the nominative case in German.
There are no objects in this sentence.
Since it is rotes, is the "ein" implied?
No; it's not "implied". Using ein here would be simply wrong -- Wasser is (almost always) uncountable.
Mixed inflection (after ein) and strong inflection (without any preceding determiner) are the same for neuter nominative -- because ein has no ending, the adjective has to show the gender and case just like when there is no determiner at all.