Its better for everyone (including you) if you reuse your previous thread for the same project rather than starting a bunch of separate threads.

1) Repeated tiles 9 times. Placed the Star Demon sprite on for reference.
2) Desaturated the nasty green colour out and removed all the ugly gradients.
3) Balanced the values. Keeping contrast low as is the general rule with backgrounds.
4) Added some various subtle hues. Keeping saturation low (lower than sprites anyway) as is the general rule with backgrounds.
Tiling is fine anyway but lined up like this you can easily tweak anything you want.
Your tile size is 32px, probably makes more sense that this is 4x16px tiles.
Anyway now you have a usable background tile. Not the prettiest thing in the world but at least it doesn't induce vomiting.
Right now let's focus on the outlines.
Don't discount advice just because it doesn't relate directly to your question.
We should all be wanting to absorb as much knowledge from each other as possible.
the Sun is pretty much overhead at all times
Consider offsetting your sun to the left (which I prefer) or right by say 45 degrees so you can define forms by rendering rather than everything appearing flat.
Refer to the Star Demon's rendering.
Do the rocks need to be more consistently sized?
No, you have a pretty good balance.
Too consistent and its boring.
Too inconsistent and the pattern will be too obvious when tiled.
The guide I was following said to black out a bunch of areas between the rocks, because rocks are not evenly spaced, but I chose not to for whatever reason.
There's endless ways to do it.
Next time you design some tiles, consider giving your ground tiles lines a prominently horizontal feel.
And go for vertical lines for vertical surfaces like walls.
This is one trick to help define surfaces.
I'm afraid to add the dithering in the bottom left, because like I said, I'm on the fence about where to go with color pallette (I'm opting for as few colors as possible, although not sticking to 16 or 256)
It would be useful if you could answer my previous question on getting some details about your color blindness.
You should check out this link below, but my advice is not to get caught up in dithering at this stage.
Limit your colours but I really think you should get more practice in and make a bunch more usable tiles rather than worrying about dithering and anti-aliasing or your just going to go around in circles.
http://wayofthepixel.net/index.php?topic=1025.0I plan on making this the main tile.
Don't fill the screen with one tile.
I think you should move on immediately and make 2 or 3 variation tiles (e.g. in some parts there are like shallow holes in the ground rather than every rock popping up).
You should consider some other surfaces like grass and/or dirt, and the various transition tiles including 45 degree angles, corners etc.
Try working on a mockup in the res of your game and draft in some other stuff such as rocks to place around the screen.
Don't spend forever on each thing, just block it in to get it started and repost.
This is the quickest way to make some progress rather than getting too caught up in trying to perfect 1 tile then realising much later that you wasted heaps of time...