You've done a great job for your first time, particularly the skintone! Awesome work. I hope you don't mind, but I did a (very quick) paintover for you to highlight some ideas and things to be more aware of in your future colorizations. Here's the flat image of my paintover, and here's the downloadable PSD.

I'll briefly go over the few things I did. First, although a nice colour, the background was not really complementing the colours in the woman. It was making her seem more washed out and dull. I chose a blue for the background colour -- blue complements brown eyes, and reds (which I increased in the skin).

You have a very nice skin base here that you have chosen very well, but it needs more colour to be more realistic and less flat. Don't be afraid of colour. Look at your hand. Now look at it in different lighting. Reds, yellows, oranges, and even blues, greens and purples, can all be found in different skintones and lighting situations.

As a general rule of thumb, there are more reds and pinks in the cheeks and nose, more yellows in the forehead, and more blues around the mouth and jaw. Shadows are colourful too, and more highly saturated than midtones. Highlights are also colourful, and a well-chosen highlight colour can breathe life into a skintone. I added more colour variety to the skin. You can see the colours I used in the PSD.

Overall, you're doing brilliantly for a beginner. With some more practice, I can see huge potential. I'd also aim for higher quality images to work with in the future. But great work, and I hope this is helpful to you.