PHOTOSHOP TUTORIAL OBJECTIVE: You will learn how to use curves and several other tricks to adjust lighting and color in a portrait in order to bring it to life. The technique shown in this tutorial is useful for beginner to advanced levels and can be accomplished in any version of Photoshop that uses adjustment layers, although the screens may look a little different.
You can click on the pictures to see them larger in a new window. I write these tutorials with my Beginning Photoshop students in mind, so they may seem overly simplified for advanced users. If you have a question, please email me using the text link at the top right of the blog. 
NOTICE: Although the techniques shared in this tutorial come from multiple sources (which have been credited), the wording and images used in this tutorial are my own intellectual property. You may feel free to share or link back to this tutorial, but you may not republish it without my express permission or claim it as your own work. 

In this tutorial, we are going to take a look at one way to use the curves feature in Photoshop to brighten up the color in an otherwise great portrait of my sister's dog, Taylor. Then we will use a other quick techniques to further enhance the image. The photo above was taken by my cousin Brian, who works as a professional photographer and videographer in Salt Lake City. We took it outside at a local park, using his high end camera with two hand reflectors. It was pretty late in the day and the light was going. Brian got a great expression on Taylor's face, but the color is really dull and the portrait lacks "oomph!"
Step 1:
First, we will need to apply an overall curves adjustment layer to correct the slightly flat color. Curves are used to reset the darkest, lightest and neutral gray point of the image. Photoshop will then globally adjust all the colors in the image — correcting lighting and color cast problems along the way.
You could just go into the curves palette and guess where the points should be, but if you follow a tip I picked up from Photoshop User's July/August 2000 issue, Photoshop will tell you exactly where the lightest and darkest points are. (I know that seems like ancient wisdom – but trust me, it still works like a charm.) First, create a new threshold adjustment layer. (At the bottom of the layers palette is a little half black/half white circle icon. Click and hold then choose threshold from the pop up menu.) Your image will turn into this crazy black and white map. Drag the slider all the way to the left to find the blackest point in the image.

Finding the lightest point in the image using a Threshold Adjustment Layer.

Most of the black will disappear out of the image. If it all goes away, slide back to the right slowly until some comes back. Select the eyedropper tool in the toolbar and holding the shift key, click on one of the black regions. It will leave a little cross hair marker that says the number one next to it. It is not necessary to be really precise on this, but if you like you can zoom in to see better where to drop the point.

And now the darkest point.

Then drag all the way to the right to find the lightest point. Some images have blown out highlights — areas that have turned white because of being too close to the flash, a very bright light source or because of overexposure, so use your judgment here to select what SHOULD be the brightest point. Again, shift click with the eyedropper to make a sample point.

After clicking the lightest point in the curves adjustment layer to reset the lights in the image.

STEP 2:
Click cancel in CS3 or throw the adjustment layer away in CS4/CS5. You only needed the sample points. (It may appear that they have vanished, but as soon as you create the curves adjustment layer, they will return.) Make a new curves adjustment layer. Click the WHITE eyedropper on the left hand side of the curves palette. Then click the the sample point that was the white one. Then click the BLACK eyedropper and click the light sample point.

After clicking the darkest point, the colors now appear more saturated and evenly distributed. But we're not done yet!

Then click the gray (center) eyedropper. Now comes the bit of guesswork. You need to find something in your image that is as close to neutral gray as possible. Since we are working in RGB, 127 is the middle of the RGB scale of 0-255. We need to find something that reads as close to that as we can. You can use the Info palette to help you locate a place to click. (Window>Info) You will see two sets of numbers. Look at the RGB scale only. The set on the left is what the color reads now, the set on the right is what it read before the curve was introduced. Look at the sets that shows what it reads now.
If your image had a strong color cast to begin with, look for the 127 on one of the other channels. If it was blue tinted, look for a 127 in green or red. You will not find a place where they all read 127. This takes a little patience and you may need to click around some until you find something that works. Look for gray clothes, metal, or concrete as a good starting point. As you click, watch any areas that are white and skin tones to make sure you are not introducing a color cast. By now your image should be looking a LOT better.
BONUS TIP: Select the eyedropper tool and use shift + option to clip out the markers once you no longer need them. They do not print and you can certainly skip clipping out the markers.

Wow! look at that! It took me 4 times as long to explain it as it does to do it. Once you understand this technique, it is a 2 minute fix.

So, why does this work so well .... and so quickly?
The answer is in the histograms shown here. Look at the original histogram I have inset into the image. In very basic terms, this is a bar graph representation of the distribution of the color, highlights and shadows in the images. Notice how the colors are all bulked toward the left side (the shadow end) of the original? This tells you what your eyes already see — the image lacks proper highlights and most of the color is in the mid-tones toward the darker end, making the image look flat. Now look at the adjusted histogram after the curves layer is applied. The colors are now spread out all the way across the graph and they all overlap more evenly. When we reset the light, dark and neutral gray points, Photoshop adjusted all the colors in the image around those new points. The lighting has evened out and the warm tones came up more strongly because we corrected the neutral point.

And now... for some Enhancement techniques!
Okay, Taylor looks good, but the grass looks dull because of the low light. Let's green it up a bit, shall we? We'll apply a second curves layer with a mask to make the green more vibrant without causing our doggie star to look "nauseous." Create a new curves adjustment layer and switch to the green channel, using the drop-down menu. Click the center of the curve and pull upwards until the grass is green enough for your taste. Then using a soft brush, paint with black on the automatically generated layer mask over the entire dog to block the green effect (in the layers palette, click the white square that just appeared next to the layer thumbnail)

The grass is greener alright, but you'll need to use a soft brush to mask the effect off the dog.

Check out the thumbnail of the mask. Black paint on the mask HIDES the effect of the adjustment layer. White area allow the effect to show through. Shades of gray are partially transparent.

Next, let's overall lighten just the dog. By making the dog the lightest thing in the image we are directing our viewer's eyes to the item we want to be most important. Our eyes naturally fixate on brighter things. So, we create one more curves layer following a tip I picked up from Katrin Eiseman in a video she did that was included with retail copies of Adobe CS3 Design Premium. Don't make any adjustments to the Curve. Simply switch the blend mode of the curves adjustment layer to screen. The entire images gets a lot brighter ... but see how the fine details are preserved? Personally, because of that, I feel this method of brightening is more universally useful than Brightness/Contrast and other techniques  Another benefit of using adjustment layers with a mask, is that you can control the strength of the effect through the layer opacity and where the effect is visible and apply blend modes.
Adjust the opacity of the adjustment layer until the dog is brightened but not blown out.
You can also mask off portions as I did here to literally "paint with light". I've chosen just to have light bathe Taylor's left hand side.
To finish off the effect I have created a vignette - a darkening of the edges of the photo. This further establishes Taylor as the image's focal point. To do this, use the eyedropper to select a very dark color out of the image. I selected a dark green in the grass. Create a new layer and fill it with the dark foreground color using Option+Delete (on the Mac, Alt+Del on a PC). Then use a soft brush on the eraser tool to remove it from the center of the image. You can blue the layer if you like. Set the blend mode to multiply and adjust the opacity of the layer until you are happy with the effect.
And here's our final portrait.
Total time: about 10 minutes.