I have no idea how to measure "colour correction". But if you mean how much the colour is changed from the original in Photoshop, it's very little. I just reduced the magenta cast in the shadows. The sand and water were very purple in the RAW, I've made it more blueish...
I asummed it could be done on a very scientific mathematical level. Its a great shot. I wish I knew how to change the color of things realistically. I have a photo in my gallery where her face is red while her body is like an ivory white. I suck at all styles of photo-manipulation.
oh, it's something you need lots of study and practice. But there's lots of free material in the internet, no secret at all, especially when it comes to portrait retouching... To keep things realistic the key is, no matter what you do, do it in small increments and keep comparing results. It's very easy to loose sight and overdo stuff. To make local adjustments in a photo, like just in the face of your model, you need to learn how to work with layers and masks. Actually, it's something you'll need to know to work with in almost every photo.
But, when you do your research, do look for "photo-manipulation", it's something entirely different. Search for post processing techniques.