Important! The center pieces are part of the core and subsequently cannot move relatively to each other. For that reason they are already "solved". The solving process is actually bringing all corner and edge pieces to the "already solved" center pieces (meaning there are only 20 pieces to solve out of the 26). For example, the blue center piece will always be opposite to the green center piece (on a standard color-scheme cube). It doesn't matter how hard you will try scrambling the cube, it will just stay that way.
For decorative purposes, a colorant is typically added to the plastic. The pieces of a Rubik's cube are typically black. During production, colored stickers are put on the outside of the cube to denote the color of a side. The plastics that are used during production are supplied to the manufacturer in a pellet form complete with the filler and colorants. These pellets can then be loaded into the molding machines directly.
If there are no more edges left on the top layer, then they are probably either inserted in the right place but flipped, or inserted in the wrong place. To get an edge out of somewhere it shouldn't be, just insert one of the yellow edges into that slot. This should get the edge out and on the top layer, ready for you to use the above instructions to insert correctly. Rubix Build