The problem with your background color involves understanding some color theory. To be honest its one of my weaker subjects but it is so very important. Anyway... I'm just going to copy paste a bit on a document I was writing for this section. I've been slacking around here but this is a small peice of my framework document from which to expand into specifics.
paste:
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This is by no means a complete explanation as color theory is a complex subject. However I will list a few basics and related topics to consider when employing color theory in your game art.
Warm (yellow-red) colors & more saturated colors will naturally come to the foreground while cool (blue-green) colors & less saturated colors will recede into the background. The same is true of contrast. More contrast comes forward while less recedes. Keep this in mind while designing your foreground, background layers and also elements that require the players attention.
Typically your background layers should have less saturation, less contrast, less detail and often times, but not necessarily use cooler colors than your foreground layer. With these factors we are creating a sense of depth while putting our attention on the player and what he/she needs to see & do. Combined with parallax scrolling the illusion of depth can be quite powerful. You should study some 2d existing games to get a feel for this. Plenty of screen shots can be found through google images.
Note: If the end user is constantly straining their eyes to find elements of focus from the background or surroundings it will become a frustrating, unpleasant experience. If a game's graphics make the goals unclear or worse causes eye fatigue how long would you keep playing it?
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http://poynterextra.org/cp/colorproject/color.html - Flash tutorial, probably the best I've seen.
http://en.wikipedia.org/wiki/Color_theory - It's a bit dry ad technical but still full of usefull information.
Looking at your last screen shot... the vines, which are in front of the trees are very hard to see against the tree line. Try using a brighter or warmer green on vines. You could also add some blue into the tree green.
You have an issue with your objects appearing flat. This is because you know no defined light source. Typically in platformers you will find that the light source comes in at a 45 degree angle from the top left. For the tree trucks that means a spotlight close to the left side and a shadow on the right. You may want to cheat in a secondary lightsource to highlight the far side of the tree trucks. Getting the lighting right is more important than adding detail. It may be a 2d game but dont think 2-dimensionally... even in cartoons there is a simplified light source.. light and shadow give form.
You've added a lot of texture to your trees (leafy part) in order to mak them more interesting. However they are still flat. Again this is because you need a defined light source. If you are going for a realistic approach this will be kinda tough however with simplified lighting (think of the green areas as somewhat sphereical) the light would spotlight in top left with shadow on the farside.
While not perfect by any means you can look at the forset tileset demo. Even there, like you, I applied a texture to make them less bland. However, there are 3 shades of green in the leafy trees... spotlight, ambient, shadow so to speak.
In beginner art classes/school among the first things you do is build a color wheel and draw/shade cones,boxes and spheres. As boring as that can be these fundementals really effects everything you create.
Your animation on your lizard, the walk cycle could use some work. While I'm tempted to get deep into that its not the most pressing matter and not even a bad attempt. There was a thread I would link to on the old exboard forums but I think those got damaged? Anyway hope this was useful. =]