My concern with the Sparkling Seaside shot is not the palette, but rather that it looks zoomed out when compared to Polar Peak. In PP, the walls have depth, the floor (that is to say, the snow) is large, and Sonic actually ends up looking kind of small. In SS, Sonic looks closer to a natural size, but the walls and floor look pretty flat in comparison. Making the grass thicker and the edge pillars wider (and perhaps even shading the parts of the wall texture immediately adjacent to those edge pillars) would go a long way to making the two tilesets look like they're from the same game.
(It also has issues with repetition; the trees in the background tile pretty obviously, and there aren't any decorations in the wall texture at all, but the flatness is more glaring.)