I agree with the statements above, 50f is darned cold for Kolsch yeast. 62f is a happy medium for me between profile and speed.
I have been playing around with kolsch yeast lately at warmer temps (66-68f, allowing to run as high as 70f if it wants to) with some very interesting-in-a-good-way results. It won't make a to-style kolsch at those temps, but it is creating some really interesting beers with an almost farmhouse character.
To answer your original question, I would bottle condition at 58-60f for at least two weeks, and then "lager" just by storing the bottles in the fridge for another 3-4 weeks. It's a long time to wait for a relatively small beer, but you'll reap the rewards in the refinement department.
If your yeast is actually working at 50f, be prepared for the fermentation to take a LONG time. Unlike the more common american strains (WLP001 for instance) kolsch yeast has a much more linear and long attenuation schedule. If you cold crash too early, you'll be well high of your intended FG.
Cheers, and I hope it turns out great!