Translation:I have another car, but it is blue.
Em inglês, sempre é bom incluir o pronome depois da palavra "but", o que não ocorre em português.
To expand on that, grammatically Portuguese (like Italian, Russian, Arabic and many other languages) is "pro-drop" while English (like German, French, ...) is not, so you always need to have an explicit pronoun, even when it's just a dummy one like in "it rains".
It's preferable. As well as in Spanish, "indefinite article + outro/a" does not sound good.
would it be better to say
i have another car === eu tenho mais um carro?
outro carro to me sounds like you are comparing two things already mentioned, whereas this sentence to me sounds like they are just now indicating the existence of an additional car.
They are the same: eu tenho outro carro = eu tenho mais um carro. Both show there is another car not previously mentioned
Just to add to Paulenrique's answer: it could also mean what samosborn88 is thinking.
"Você tem um Ford?" (do you have a Ford?)
"Não, eu tenho outro carro" (no, I have other car - a car of another brand)
Would it be right to translate: "I have another car, but in blue"? In english it would be little smoother than the provided translation. But i do not knwo whether the portuguese sentence could be translated that way.
Maybe "eu tenho outro carro, mas na cor azul" (a sentence probably used by a salesman).