if you go pick up say 3-4 more and put them in, you are almost assured a pair. Then as they pair off, take the others back to the store.
With all due respect, I would use some caution with this approach, due to the fact that an entire brood of krib fry can end up all one sex or all the other -- it is influenced strongly by the water pH they are born and raised in. One lfs, for instance, had a whole tank of kribs that were all males. Many were developing beautiful finnage, but not one had a colored belly.
Your lfs could have had all females, but I kind of doubt it because you say that at least one of yours is very colorful. I assume you mean the pink/red/plum colored belly. I have two kribs, one I am certain is a male, and one I believe to be a still slightly immature female. They are not currently "housed" in the same tank, and neither of them are the slightest bit colorful in their abdominal region. And yet, at the lfs where I bought the female, there were at least two fairly mature and aggressive males in what I call "breeding color", meaning that I think it likely were females in the tank with them due to their coloring, but I believe the females were as yet too immature to show their colors or respond to the males, so there were no pairs.
My other problem with the "buy a bunch and let them grow" approach, which I've read in several other places on the web, is that I don't necessarily
want a bunch of kribs, especially not males, so then what do I do?
Not all stores are friendly about taking fish back. I know that the one where I bought my male really frowns on it, and where I bought my female, they have a 3-day return policy.
So, if your fish is colorful, that may be your female and as Pufferpack suggested, your male may be less mature. Or perhaps it
is a less developed female and not a male, but time will tell.
Depending on your situation, you can either:
1) buy a few more and see what happens, or
2) wait a while and see what happens, or
3) take one back and look for one with an elongated dorsal fin that has a spot at the end of it, more of a spade shaped tail, and a less pronounced abdomen. See photo of my male
here
HTH.