Ryona>
I really like your mockup. It has a cool techno-feel and looks mostly consistent, which is not easy. Did you intended use the nes restrictions? If not, ignore my further techno babble.
The nes don't have any layers, thus a background like yours wouldn't work. I counted the colors of some of your tiles and on some you have 5 or even 6. For the tiles itselft you have mostly a correct amount of colors, but don't forget: Background tiles have the same restrictions as sprites. You have 3 different colors and and one color is always shared throuh all palette rows (4 rows for background tiles and 4 rows for sprites). In sprites your 4th color is always transparent, meaning, you can see the background tiles through it. The same to the background tiles. You can use 3 colors, but one color is transparent. But now you can't just add a new background with tiles. Think of a background color you paint on. Sometimes the background color can be part of your tile.. thats where you use transparency. What you have as a far away background should be one color. If you want to have such a background , then look out for the borders of your platforms where you tried to use the 4th color.
I made a small paintover to show some places where the nes restrictions were brocken. You can ignore what I did with the sprites, because I don't know how you aligned them to the 8x8 grid, but maybe they have some errors too.
