Hm, you mean the sun needs to be brighter in terms of saturation, or value?
I tried to make it a bit brighter in this edit. Also, I did a very minor edit that I think really helped the perspective of the coaster in the foreground... I can totally see that the perspective is off, but I don't really see how it clashes with the platformer perspective. Like, its supposed to look like the coaster is continuing to spiral towards and past the player?

The stand generally has the amount of detail I want to have for all the foreground elements, while the background will remain in silhouette form. So I think that it won't stand out as much when I finish fleshing out the foreground (which is what I'm trying to do with the path bricks, coaster in the foreground, etc). I agree on the dithering- I don't like dithering.
I really don't know how to make the player stand out more. :/ If I make him more saturated or add outlines, I'd have to change all his animations too. I could decrease the saturation of the background, but I really like how it looks now. I'll probably end up doing that. However, the player does stand out way more in action.
EDIT: A darker background does make the sun look better and is generally easier on the eyes. I'm experimenting with some colors now.
EDIT 2:
I needed a video for my weekly dev blog post, and I couldn't wait to see it in action so I went ahead and implemented it. I'm pretty pleased with the results. I can always just replace the images later now.