rocknbil - 5:03 pm on Jun 20, 2012 (gmt 0)

I can't "get over it", if it does not look good it compromises use experience... I expect most users would use default setting for his/her graphics card.

This is what everyone is telling you - "the default" may be limited to 256 colors and the one you've chosen is outside that color gamut. "Most" users don't even know what a graphics card is.

The best way to ensure a more even color is use a small background gif, jpg or png.

If the color gamut on a given system is limited for styled colors, it will also be limited for an image color, it's still rendered by a browser.

using image increases download size and loading speed...

You can use sprites or a 1 X 1 pixel image. But it's not likely to help, see previous. :-)

If the color is shifting over different browsers/operating system, you need to be quick on your feet and find a good compromise that doesn't interfere with your concept of "user experience." Honestly, the shade of blue a user sees isn't going to sway them one way or another. Your choice may only be a few cells off from web safe. Restrict your palette to web safe as tedster suggested and give it another go.