How to make the top part of the render less saturated/more pale?
The Mesh needs more of a pale color in top part to match the background. How?
You can do in compositing nodes.
Here is an example:
Node Hue Saturation Value can make your image more/less saturated, but you need to add a mask to specify on that part of image the effect will be affects. You can draw a mask, draw an image or like in my case, use a box mask node and blur it.
If you familiar with masks, you can draw a mask with feather, and use it, it gives more control of areas witch must be saturated:
May I suggest a simpler method?
Add a Texture node to the compositor.
Now head over to the Properties window, the Texture tab is the one we are interested in. After clicking the large New button, you will have a texture. First off change the Type to Blend (a gradient texture). Then down at the bottom set it to Vertical. (I find Quadratic is a nicer Progression, but completely optional.)
Back in the Node Editor, specify the texture you just created in the awaiting Texture node. Just click in the empty field, and a dropdown of the texture(s) will popup.
That is it. You now have a gradient in the compositor.
To further refine the gradient go back to the Texture tab in the Properties window and enable the Ramp check box (encompassed in the yellow box in the image above.) With the Ramp enabled you can now tighten the end handles, add more color stops, reverse the direction, and so on.
You can see I moved the black handle towards the middle. That squashed the gradient, moving the start of the fade up, thus giving me more of the original image in the composite.