Aww, no worries. Grass is really hard.



Here, I removed the grey-things. You don't need outer aa-ing, especially since there really wasn't going to be a point for the white background. I also removed the grass because I planned to help you refine the tree. :3

See, you have three colours in which you barely use, and you can remove them without losing any information. So with this second image, I removed them.



Normal greens... they're like, so five minutes ago. ;P So, to make things look more interesting, I changed the hues around a little.



Then, as mentioned before, you don't need those holes. So I filled them in and such.



I didn't do anything to the trunk because I'm afraid I'm not very good at doing trunks, so I don't want to say something is wrong without knowing all that much about it myself. Anyway, you can use as much as you want from what I told you, and welcome to the forums. :>

Dithering

Onoez, you used the spray can! That's alright, some of us make that mistake at least once in our lives. Now, I picked a section of your work and dithered [semi]correctly, as you can see from here. Sure, it seems difficult, but it isn't really, after you get the hang of it.

Here's an example of dithering between two colours- in which I cheated and used a monochrome bitmap, but hey, this is just for example.



There's just so much you can do with only two colours, huh? ;3

I'll be honest, it'll seem mundane. You will want to take frequent breaks. But it does make the piece look more uniform; The spraycan does dither, but it's like... speckle dither. You don't have complete control of each pixel, and that's what pixel art is all about.

Here, I removed the section I dithered [semi]correctly for visual sake. o:



Looks a bit better, doesn't it?

Colours

I can't really see the two top colours separately, from first glance. That's not what you want, ever. >D You want to use as few colours as possible, and because of that, each colour should at least seem like different colours right from first glance. Since I did this quickly, I'm afraid I still didn't do the job correctly, but the colours do look a bit better.



Stars

As Helm mentioned, 'stars are in constallations, not uniform patterened grids'. That doesn't mean that where you placed the stars are wrong, exactly, but the way they're placed does make it look like a grid. To mask that, you draw smaller, bigger stars around it. Again, quick job, but does make it look better.



Remember, stars have different sizes and intensities.

To get this done, I had perhaps... one colour more than you? I dunno,

Now compare the section I did to the whole picture.

Jus' trying to help. Take from this what you want. I do suggest, even though you said you won't edit, to redo this piece in a smaller scale. ;3

Edit: Tried the colours again, added more stars to make me look less lazy.

   

It was meant to look blinding, but I have to admit it looks a bit awkward. I did like the red, so I'm not very fond of this second one I tried, but hey, until I get the mistakes fixed, I'll use it.

And what may that pallete be? It's those 50x50s with striped bgs, if I'm not mistaken?