@Shaz: In the past these have been put both here and in Art, Literature and Music. For now I think either is fine.
@boldpaste2: Pixelize filter?! *shudders* No. Yuck. That's worse than using the brush tool then reducing the colors. The whole point of pixel art is precision and purposeful decisions. You can't fake it, when you try it becomes really obvious.
@tearsofthenight: Yeah, using 4 colors is important, it's probably best to use the same ones (either grey or green) as the original gameboy so you don't have to worry about getting proper contrast between them. For anyone who's looking into making pixel tilesets I usually recommend Pyxel Edit, it makes things so much easier, especially if you're making tiles that need to repeat seamlessly. When using such a limited pallet you have to pay really close attention to positive and negative space. It's really easy for things blend into each other and become and unreadable mess. You'll need to start with a mockup, not the individual tiles, so that things can be visually balanced with each other. Once you get some stuff done you should post in the Resource WIP thread, feedback is really useful in things like this. I'd also recommend looking up games that use similar art to what you want and use it as reference. Get multiple so you're not too heavily influenced. References are really helpful in figuring out how other people have faced the same problems you will be when putting together your tiles.
Edit: Almost forgot. GB tiles are smaller, 16x16 or 8x8. I suggest working at 16x16 then doubling the size before you import it. There's a program called MagnifyPNG that you can find on the forums that can be really helpful for that.