For a 1-gallon batch I use .1 oz of Columbus for bittering. My LHBS sells hops in two oz. packages so I would put away 1.9 oz stored for future purposes. Or I could use .15 oz of the Northern Brewer that I already put away for future purposes one or two batches ago.
Maybe I'll use up the Columbus soon. Maybe I won't. I am definitely *not* the kind of guy who can plan ahead with ingredients.
So the question is, is columbus so distinctive that I really want it, or is hops is hops (especially bittering hops)?
Yeah if the northern brewer gets you the IBU's you want I'd just use up what you have. On the other hand, it never hurts to have some CTZ around. It's high AA and usually cheap so good for bittering but I also like it for flavor and aroma in American ales.
I think the northern brewer will be fine in that quantity as a bittering hop replacement for Columbus (CTZ). Heck brew two 1 gal batches with the different bittering hops and see if you can tell the difference. I bet the amarillo will "cover up" any particular bittering hop flavor.