The 256 colors is only part of it. An uncompressed ( or compressed at quality 11 or 12) file with 256 colors still looks pretty good - it it the further size reduction/compression for web use that does it. One way to hide it a bit is to add noise, believe it or not. That tends to even out the blends.

Or you can add a slight gaussian blur to the sky area. That too will even the blends a bit.

Has your monitor been calibrated? You probably need to do this so that what you see on the screen is very close to what the output of the printer will be.

One of the basic things is that we often have our monitors set at too high a brightness level resulting in prints that are too dark. Sometimes and I emphasize "sometimes" that is enough to normalize the prints. If you are saying that the color is off, then you will need to do a calibration. Google it and you will find many systems available to take care of it.

If you are using Canon paper and inks then make sure that you are making the correct choices for the paper type when setting up the printer. If you are using non-Canon paper, then you may need to have your post processing program do the color management (rather than the printer) and install the proper profile for that printer/paper combo. Many paper manufacturers have them available on their websites.