As far as I know there are four different materials out there. Regular steel, well, it comes with a few different coatings, but still the same steel inside, then stainless steel which has outstanding qualities, but is a pain to do yourself, and then it is what I call copper and the fourth is what I call copper nickel. Both there copper ones look very different, the one I call copper looks like pure copper in color, and the copper nickel has a totally different color. However, both these are copper alloys, I think one has some 89 percent copper and other has 90 percent, and I think both has something like 9 percent nickel, and one has 1 percent iron in it, and the other has 2 percent. I have no idea if this tiny little difference is what causes the difference in color, but among car people in Norway at least they are referred to as copper and copper nickel. As far as I know both of them has a burst pressure of around 19000 psi, which is over 9 times the maximum pressure you will ever get in your brake system. It is a little less than steel which have 19500 psi. A brand new steel pipe never burst in a brake system. But an old one might. A brand new copper pipe does not burst either, and neither does an old one. If a brake line burst when new, either steel, stainless or one of the copper alloys, it is not because they are close to the brake pressure in store ght, but something has happened to it, a too sharp bend, wear against a sharp edge or whatever. But that goes for any pipe of any kind and for any purpose.
Bill