"Die Zwiebel ist rot."

[Mr.P.Kiwi]

Why a feminine noun gets a masculine adjective?

[mizinamo]

There is no masculine article or ending in this sentence.

Predicative adjectives (basically: those after the verb "to be") have no ending at all.

So you would say der Apfel ist rot, die Banane ist rot, das Fleisch ist rot, die Tomaten sind rot -- no matter what gender or number is in front of the ist, the adjective has no ending at all.

[Bill_Tappen]

So hard to understand her pronunciations sometimes!

[hoomau]

OK. So when do we use a sein verb and when do we use a haben verb for color?

Welche Farbe hat das Hemd?
Die Zwiebel ist rot. Deine Jeans ist blau.

[fehrerdef]

If you talk about the colour as a property of an object directly you use the construction
X ist rot" (form of "sein"), literally "X is red"
If you explicitly talk about the colour (not the object itself) it is
"X hat die Farbe rot" (form of "haben"), literally "X has the colour red".

Though the second sentence would be very unusual, if not wrong, in English, the construction should be comprehensible.