Hi Maureen, it refers to the different colour PVC some smurfs are made from, most being either blue or white but there are many other base colours used too.
A good example is the Mermaid smurf, she comes in both blue and white PVC and can easily be seen by looking at the bottom.
Some figures are easy to tell as there might be an unpainted area on the figure but others are more difficult to tell as they are completely painted. I have noticed recently that quite often a WG smurf might have a blue PVC base but the HK smurf will be white and then completely painted.
I am in the middle of adding descriptions to Blue Imps and when I have noticed a difference I have pointed it out. I am not 100% sure I have spotted them all but I know there are quite a few that I have done. I have though only mentioned the PVC colours when there are 2 or more being used for the same figure.
A good tip it to look closely at the face and hat to see if they have been painted and also at the bottom of the feet to see of there is paint missing.
Here are a few that I have noticed:
Devil - Pink / Red
Indian Smurfette - Brown / White
Goodluck - Blue / Green
Papa & Pizza - Red / White
Cowboy - Blue / White
Sauna - Blue / White
Violin - Blue / White
Amour - Blue / White
Surfer - Blue / White
Farmer - Blue / White
Mermaid - Blue / White
Papa - Blue / Red
Rugby - Blue / White
I am sure there are plenty more, these are just the ones I spotted straight off after a quick look. I haven't gone through them all by any stretch.