The Pentax K series, is what people would say, erratic or weird metering.
Some says, it's perfectly normal, while Nikonians would say, it's badly under exposed.
I have the K10D myself too and I always crank it up +0.3EV for evening/night shots.
Just like the review of K10D by Phil Askey, if you are not the kind that wanna fiddle with RAW, you might as well stick with C&N.

Somehow, when I look at these K100D's shots, the behaviour is probably the same. But then again, I've only made 5 shots using green mode, but those are indoor. Perfectly balanced though.
Anyway, any underexposed can be fixed with the PP Lab software, no worries. Just make sure you snap in RAW, else there's no turning back.

Keep up the good work!