I hate to sound like an a$$, but you did in fact underpitch pretty severely. I have had very bad experience with underpitching Kolsch yeast specifically in the past. My underpitched Kolsch tasted a lot like apple and turpentine, and just plain undrinkable -- I ended up dumping it. I predict there's about a 75% chance that your beer won't taste all that great, and you might even need to dump it as I did. Next time pitch more yeast up front. In the long run, you'll be glad you did. Try
www.mrmalty.com for a nice yeast pitching calculator that really works and makes sure you always pitch the right amount up front.