I see a wrong workflow here. Pixeling and fully detailing body parts one after another instead of progressively working on the whole piece is a sure path to failure. There are 2 ways to start a piece: either draw the whole line art (as apparently you started doing from your first post) and make sure it's good before you begin shading and coloring; or block out very rough color patches and refine them progressively (or combinations of these). Don't even think of dithering at this point!
Also I don't think working on such a large canvas is recommended. Pixel art being about control of individual pixels, the more there are, the harder it is (and soon hopelessly so). 100x100px is 10,000 pixels and that's already a huge lot; 500x500 is 250,000! No way anyone except the most experienced pixelers can tackle this.
Sure, you won't be able to show every individual scale, but that's not the point. Pixel art is all about suggesting.
Dragons happen to be quite ambitious to draw and pixel. I'd suggest to
study how others did it, and seriously consider the 'simpler' option of a small, cartoonish, scale-less dragon such as these cuties:
1,
2.
Next, think about colors and light.
You already have 48 colors (!) there, most of which are indistinguishable and cover a few pixels. Try to build a consistent palette and stick to it (although it's fine to tweak it). Anywhere between 8 and 32 colors should be a goal, according to the style you're looking for. Consider using existing palettes such as Arne's or Dawnbringer's.
Shading: define a light source! Presently there is none. Using references is good for this, BUT you must make sure you understand the light in the ref. Johasu's link isn't an easy one as it has at least 4 distinct sources: direct sunlight but not everywhere; diffuse (blue) light from the sky in the general direction of the sun; dimmer diffuse blue light from the opposite side; diffuse greenish reflections from the ground. Try to see how each of these affect the image. Of course in your piece you'll create a much more simple light: one direct source is enough for now.
At this point you should probably experiment with simple small tests of these various issues, and start again on this piece when you have a better grasp of them to pick a direction and stick to it. Reading some good
tutos might help too
