In the Muppet Movie Sam's feet are blue. In Muppet Treasure Island they are clearly yellow and have been so every since. There's a history of confusion concerning the color but the licensing model sheets that Henson provided Palisades clearly noted yellow feet. This is how they want him and Disney seems to agree (or not notice).
Technically, I think Sam's feet are blue, but kind of like the blue-nosed, green-lidded Gonzo of the Muppet Show's first season, Sam has evolved. Visually it makes him pop a little more. But as far as consistency, his feet really should match his beak.
So after that long explanation, Sam's feet are originally blue like the rest of him.