1. It is green and yellow.

2. It is green but yellow.
3. It is green but not yellow.
4. It is green or yellow.

(Which one is not grammatical? Thank you!)

#2 doesn't make any sense!! The others are fine.

The grammatically incorrect sentence is sentence number 2: "It is green but yellow." In this sentence, the use of "but" is illogical because "but" suggests a contrast or contradiction between two things. However, in this sentence, both "green" and "yellow" are used as descriptors of the same thing.

To determine which sentence is grammatically incorrect, you can analyze each one individually. Let's break down the remaining sentences to see why they are grammatically correct:

1. "It is green and yellow." - This sentence uses the conjunction "and" to indicate that the thing being described has both the properties of being green and yellow simultaneously. This is grammatically correct.

3. "It is green but not yellow." - Here, "but" is used to introduce a contrast or contradiction between "green" and "yellow." It states that the thing being described is green, but it specifically excludes the property of being yellow. This sentence is grammatically correct.

4. "It is green or yellow." - In this sentence, "or" is used as a coordinating conjunction to present two possible options: the thing being described can either be green or yellow. This sentence is also grammatically correct.

Therefore, the only grammatically incorrect sentence is sentence number 2: "It is green but yellow."