Neutral Density (ND) filters are for reducing light to varying degrees (and nothing more). You use those when light is too bright, generally for video, when you want to bring the shutter speed down to match a certain frame rate.
Polarizing filters are not designed for lowering light (though they will reduce light somewhat just because of how they work). They are meant for reducing reflection off of certain objects from certain angles of sun. Sometimes, in reducing glare / reflection, the color of those objects seems to become more saturated. In fact, it it is not so much being more colorful as it is the color is just transmitting better because the glare has been reduced / removed.
Neither the ND nor PL filters are for use in low light. It is recommended to NOT use either filter in low light, unless you have a special case, such as lowering the shutter speed even more, or you still have harsh glare/reflections on a cloudy day (not likely).
Some filters are combinations of both ND and PL, but not all. That's why you will see some sets that have 3 filters of ND/PL, and another 3 that are ND only (no PL). You do not always need or want both.
Chris