for a porter, color is pretty easy... you can get the srm's a number of ways. Special B, on the other hand is pretty unique. It's going to contribute raiseny, dark fruit, dark but not burnt malt flavor. If you have a local shop, I'd really suggest you go in a chew a little bit (good to do for ALL malts for familiarity with flavor) to get a better sense of what is 'Special' about this malt.
if you want to use it in your porter, I'd suggest keeping the amount pretty low. it's intense, a little goes a long way. in the right place, it's wonderful..... imagine a Belgian Dark Strong.