What is the default (blue) value for AppleHighlightColor?
The original value of blue is used when AppleHighlightColor doesn't exist, therefore you can reset it deleting AppleHighlightColor:

defaults delete -g AppleHighlightColor


You can change the AppleHighlightColor with a GUI through System Preferences → General → Highlight Color.



Setting the highlight colour to Green through System Preferences:

~ ❯ defaults read -g AppleHighlightColor
0.752941 0.964706 0.678431


Setting the highlight colour to Blue  through System Preferences:

~ ❯ defaults read -g AppleHighlightColor
2015-05-21 18:47:25.072 defaults[37881:344707] 
The domain/default pair of (kCFPreferencesAnyApplication, AppleHighlightColor) does not exist


-g is shorthand for NSGlobalDomain