I never go by color alone - had a buddy of mine explode a battery in his face once by hooking it up backwards. Someone in the past had gotten cables that fit from Sears or the like and hadn't paid attention to what color went where. Fortunately, he had goggles on and was leaning away, so minor flesh wounds but nothing else.
The positive post is physically larger than the negative.
On wire colors - I had a '76 Scout II once, and the ammeter was acting a little funny - didn't seem to be indicating the charge I knew the battery was getting. So I popped the cluster off. At some point in the past, someone had done a very thorough job of rewiring the entire cluster. Unfortunately, they must've gotten a good deal on wire, because EVERY SINGLE WIRE was green - the whole cluster. I buttoned it back up and sold the truck (it had other issues as well, this was the straw that broke my back).