the first thing to try is to turn your NOOKcolor completely off. Do this when on by holding down the power button for 5 seconds, until the screen gets blurry, and it asks if you want to "turn your device completely off". Tap "power off".
Once it's off, turn it back on by holding the power button until the screen comes back on. Then go and see if you're still having the same problem purchasing apps. Let us know. (There are other things you can try, but they get more complicated.)
(I'm assuming you have a good wifi connection, othewise the Shop wouldn't let you get that far.)