Oh Dear, I am a lousy instructor, but the best way to explain would be to use GIMP as its free to download and easy to use,.
Right, if you don't have gimp google it asnd download it, its free for life, so always worth having,
Run Gimp, in thre GNU Box click, File, Click open, and select the photo you want to work with
You should now have the photo you want to edit open in an image box, go to colours on the top menu bar select levels, and a new box will open, if you click the Auto Tab (Lower centre of the levels box 9 times out of 10 it will correct it about right automatically)
You can fine tune the image further using the manual controls in there, but most of the time the auto setting will clear most of the colour cast. Click OK to lock the changes, If you want to improve the skin tones a bit more you can then go to the colour bar again and select Colour Balance, Leave on midtones and slide the Yellow Blue Slider a few points to blue, also on the shadows, its all a bit trail and error, but once you get one right the same settings should work on all other images taken in the same lighting.
Hope that helps some, and as I said sorry I am a lousy instructor,
(The above was based on a JPG image by the way)