I couldn't for the life of me get the colours to match on both sides of this one... sorry. I tried blending and blending, but it either looked worse or was just as apparent as it is now.

I'd like to ask castle how he matching colors.

You can sometimes use curves to line up colors (hint: sample similar regions and look at the histogram median, and deal with one color channel at a time). That may not help here, since this is probably a printing problem rather than a scanning problem, but it's usually worth a try.

I made a quick pass over the top to try to smooth it a bit (in low-res since it's just a quick hack: http://img105.imageshack.us/img105/921/blahzm0.jpg). Clone stamp, a large brush size (for the area at the top above the cloud, converting back to the original image dimensions, it'd be about a 400px brush); larger brushes for smooth areas look a lot smoother. Low hardness (25%), aligned off, airbrush on, opacity 100%, flow around 20% (lowered to 10% and then 5% for minor adjustments). If you have a tablet, there are probably better ways to do it.

Gradients and patterns like clouds are tricky; this is the best I've come up with so far...

It's harder further down, though, with other details to worry about. You can try ignoring them--just brush over them in a layer--and then create a layer mask, mask them back it individually, and then manually fix the edges with more cloning if needed.

Yeah I tried with curves, and blur tool, and clone etc... I couldn't do the whole picture so I gave up and thought someone here might be able to do it. I even tried using masking to adjust each side differently but still couldn't get a match.

I just gave it a try. Don't know if that looks OK to you guys.
http://muu.in/castle/castle/090212.jpg
adjust color balance of the left side: red+35, green+17
adjust brightness of the left with +4
and then lots of smudging.

midzki said:
I'd like to ask castle how he matching colors.

Probably gut feeling?
Usually I sample colors from both sides and compare the CMYK values.

Castle, looks fine to me here..nice work :D. Except that line..but i'm sure you've left it on purpose (since you were working on color adjust).

Sometimes, I meet terribly printing problem.
e.g. http://img118.imageshack.us/img118/1261/page12ff5.jpg
-_-
...don't blame me. I just recreate original colors truthfully. but i don't have a skill to fix this problem.
hopefully, there are people who have skill of fix these probrems here.
and castle, you are superb =D