Yup, nice warm water with soap will do a good job on that "grubbyness".
I've also used bleach with sucess.
No side effects to the pvc and colors yet.
Has the advantage that the white pvc will become slightly more white if it's yellowed.
I've heard of another method being used to whiten yellowed LEGO, but have bo ideia what effect it may have in smurfs, as they have diferent type of material.
And I haven't tested it, cause I've still not found the product in Denmark to try it. (or I just don't know what's called or where to buy it).
There are those who believe that life here began out there...