Salvinis are at the best when displaying their breeding colours, which could have been the case when you bought him. They do, however, change colour according to what mood they are in. He could now be displaying his 'war' colouring, which may change when he is more settled.
Something many people don't know about Salvinis can be traced back to their original collection in the wild, and which bloodlines they have been mixed with until arriving in your tank.......... there are two main types of collection in the wild - from rivers / streams and from lagoons. For some reason the lagoon caught Sals have much more vibrant colouring than their river cousins. In captivity how vibrant they are can depend on what their parents / grandparents etc. have bred with. So sometimes it can be just down to dumb luck.
I have also found if you match them with an equally abled adversary, such as a JD of the same size, that can bring them out too