So first I think your tileset looks okay, I see you're aiming for a minecraft-ish style with the graphics you have shown. You are trying to make graphics with the limitations of the NES and the first thing you did great is that your tiles are 8x8 pixels but on the other hand you're using colours which were not available on the NES. If you want to strictly follow the rules you should look up the NES color palette on google and only use these colours. What I would improve on, is that the textures look really flat, use more colours, add shading or use dithering. Add more variations to the ores because now you just changed the colours and that would look repetitivein-game.