When I go to open a screenshot that I had taken: ColorSync Utility tells me that the file cannot be opened. The thing is that a week ago I opened screenshots succesfully !
OS X sometimes changes for strange reasons the default application that is to be used for certain files or file types. It also sometimes happened to me that suddenly ColorSync Utility wanted to open files that have nothing to do with it and could, of course, not be opened by it.
To fix this follow these steps:
1. Find one of the new screen shot files that want to be opened by ColorSync Utility and click onto it once (not twice!) so that it is highlighted.
2. While being in the Finder press Command + i (= Get Info) and a small window with some file information will open.
3. In this window - under "Open with:" - where it now should say "ColorSync Utility" click onto the word "ColorSync Utility" and choose "Preview" from the list of applications.
4. Underneath of this click onto "Change all..." and confirm this by clicking onto "Continue" in the dialog window that opens.
5. Close the Get Info window.
When you now double-click onto the screen shot file it should open again with Preview.
Please let me know whether this worked for you.
PS: This is pretty much independent from your screen shot format. Very most likely your screen shot format is PNG and there is no need to change that.