The grass and roses that are red.= A and B that [are] C.
A and B that C = that is not a sentence.
Yes, in this case, the grass and the roses are red. But is it not a full sentence.
A full sentence is: The grass and roses are red.
A and [B that (are) C] is not possible, as shown below.
**The grass and /roses that are red/. is not a grammatical construction or a full sentence.
What would be ambiguous is this:
The grass and roses that are red should be cut.
To avoid that ambiguity, one would have to write:
The grass and the red roses should be cut.
So yes, the first utterance that does not have an SVP (subject verb predicate) does mean the grass is red too. Think of anime or cartoons. Why not?