'It is red' is a true statement, a fact. If you add a questionmark (?) it makes it the same as a question. 'Is it red' is a question. Hope that helps!

Is it red? is a question ,interrogative form , to ask a bout the colour of something , because you probably don't know it "It is red" is more of an affirmative form , it is to confirm that something is red .It is not a question!