Heck, I just made a helles and popped the first bottle tonight, and I figure the SRM is closer to 5-6 than to the intended and theorized ~3 SRM. I too used Castle Pilsner malt as the base, with about 20% other malt at maximum 3 Lovibond. However I think in my case it was due to the double decoction. But there could be other factors at play, including perhaps the pH although I know that was 5.2 in the mash, and now when I drink it, it tastes slightly tart. Certainly not a high pH thing. But anyway. Might be in part the Castle Pilsner malt, and in my case, the decoction just made things even darker.