It seems like with guineas you get 1 solid color, with or without pied. Instead of with chickens, you can get multi-colored chicks.
The ones without a lot of white will most likely be just Pearl Grey. The Pearl Grey with white on their faces/wings will most likely be pied. You won't know how much pied they'll have until they get their feathers.
Oh, and on staying the same color..They won't really stay with that pattern, but since that's what Pearl Grey guinea keets look like then yes their base color will be the same when they get older.
How much you can get for them depends on where you live and how many guinea breeders are around you. You should be able to get at least $2 each for keets younger than 1 week old.
Have never heard of pied, the white hen just showed up one day and just stayed on the place, we were surprised that she had fertile eggs since it seams that the pearl ones seam to run her off all the time... Thanks for The INFO TFTI