This is due to LibRaw, the library Shotwell relies upon for RAW processing. LibRaw is based on dcraw.
We've seen this problem pop up on various cameras over the years. The problem is usually solved in dcraw at some point, at which point the code migrates to LibRaw, where it's eventually available to Shotwell. It may be that dcraw / LibRaw still doesn't process your camera's images correctly; it just depends when the authors fix these problems.
What to do? First, I recommend upgrading to the newest version of Shotwell and LibRaw if possible. I don't know what version of Xubuntu you're using, so I don't know if that requires upgrading your operating system. You can find the latest version of Shotwell at Yorba's PPA: https://launchpad.net/~yorba/+archive/ppa Even if your version of Xubuntu has an old copy, you may be able to upgrade to the latest (0.14.1) through our PPA.
Second, if that doesn't work or you can't upgrade easily, you should try changing the RAW Developer to Camera in Shotwell. This is available by selecting the photo and choosing Photos -> Developer -> Camera. If you see this problem with all your RAW photos, I suggest changing the default RAW Developer to Camera in Edit -> Preferences.
The Camera Developer uses the RAW file's built-in JPEG file (or paired RAW+JPEG if that's the case). This JPEG is created by the camera itself and shouldn't have discoloration issues. You can probably try the second step right now and see if you get the right results; either way, I recommend upgrading if at all possible.
NOTE: I didn't notice until I posted the above that you're using the Shotwell Photo Viewer (i.e. you're double-clicking on the RAW file in Nautilus). The Camera Developer is only available in Shotwell's library mode, that is, you run Shotwell and import the photos into your library.