Because oranges aren't supposed to be red mon ami

The definite article 'the' in the English sentence indicates that it is talking about some specific oranges. However, 'संतरे लाल होते हैं' is a generalized statement about all oranges.

The color orange in Hindi is नारंगी (which incidentally shares the same root as the English word). You can also use नारंगी to refer to the fruit though संतरा is somewhat more common.

Fun fact: The top third of the Indian flag is not actually नारंगी. It is केसरी (saffron) colored.