The color of the sweatshirt was navy blue, a color that is often simply called “navy.” I think that you are likely being confused by the fact that Pony says
I looked at their clothes and realized for the first time that evening that all I had was a pair of jeans and Soda’s old navy sweatshirt with the sleeves cut short.
This is on page 44, in Chapter 3. Pony is with Cherry, Marcia, Two-bit and Johnny after the movie. The Soc boys see them and want to fight.
I think that you are confused by the fact that Old Navy is now a brand of clothes. But there are two things to know. First, if it had been one of their sweatshirts that Soda had given Pony, the words “Old Navy” would have been capitalized because they would be a brand name. More importantly, Old Navy did not exist when S. E. Hinton wrote The Outsiders. Old Navy was founded in 1994 whereas Hinton wrote the book 27 years prior to that, in 1967.
Therefore, when Pony describes the sweatshirt as “old navy” he is saying that it is old and that the color is navy.