Well....the fact that the screen shot also didn't turn out right leads me to believe that its not just a PS issue....PS has nothing to do with a screen shot.
Before you ditch the whole idea and go back to crayons try this as a long shot...
Check in Preferences > Performance (Or whatever the Mac equivalent is) for the GPU settings...
Try 'Basic' or 'Normal' for the drawing mode and if that makes no difference then try turning off 'Use Graphics Processor to Accelerate Computation'.
If all that fails then try turning off 'Use graphics Processor' altogether in Preferences > performance.
You haven't specified any details of your system so that's about as much detail as I can go into.....I'm also assuming by 'the very latest version of Photoshop' you mean CC 2014 as you've not stated that either.
If you DO revert to CS6 there is no need to uninstall 'the Latest Version'...you can run more than one version at a time....then if it still fails in CS6 at least you won't have to re-install 'the Latest Version'.
Regards.
MrTom.