It's important before you make any serious color corrections in Photoshop that you calibrate your monitor. What this means is that you calibrate the colors and black & white points of your monitor. I'm not going into too much detail about calibrating colors. A good site like Photoshopsupport already has a section where you can find some good answers: Photoshop Color management Setting the Black & White Points however is fairly easy and is already discussed by me on this page.

Save the following image of the beautiful island Madeira on your computer (right click on the image and select, if you use Windows, Save Target eye):

 

You'll notice that the image looks a bit flat, since it lacks contrast. There's is also a distracting blue haze hanging over the image that we want to get rid of. We're going to use a Levels and Hue & Saturation correction for that. You might ask; why not select in the menu Image/Adjustments/Auto Levels or Auto Color? The reason is that these automated corrections quite often don't give you optimum results. At the end of this tutorial you can compare the result of this tutorial with any of these 2 tools and you'll notice a substantial difference.

Let's first start by clicking on the Eyedropper Tool icon in the Tool Bar before we open any image.
Now check the options bar and notice the option Sample Size:

Later in this tutorial we're going to use the Eyedropper tool in combination with a Levels Adjustment. Before we use Levels we want to change the sample size to 5 by 5 Average.
Click inside the box to do so:

The eyedropper tool will now sample the colors in an array of 5 by 5 pixels and will calculate the average color. This will in the end allow us to achieve more accurate results.

Open the previous image in Photoshop and add a new adjustment layer by clicking on the Create a new fill or adjustment layer icon in the layers palette. Select in the menu that pops up Levels...
We're now going to adjust each color channel individually. RGB images consist of 3 channels; a Red channel, a Green channel and a Blue channel.

Start by selecting the Red channel in the pull down menu:

 

Click on the black slider (marked with red), hold down the mouse button and move it to the right until you've reached the start of the histogram:

 

Do the same with the Green channel..

 

...and the Blue channel:

 

Of course you can also change the values by using the Input Levels box. For the Red channel it would mean that you have to enter the value 25...

... and for the Green channel 52and 85 for the Blue channel.

Click OK when you're done. We now have not only corrected the colors, but also corrected the contrast of the image.

In general we place the sliders where the histogram starts. In the case of the Blue channel I've gone a bit further, which was just a personal preference; the result looked better. If you have doubts how far you should go in some cases, just check / uncheck the preview box a few times, that's what it's for.

Also be aware that these particular settings work well with this particular image only. Other images probably need different settings and sometimes you need to make a selection first to adjust only one particular area of the image.
In some cases you can use the same approach as I'm showing you in this tutorial, but by using the white slider instead, the one that marks the white point. In some cases you need to move the gray slider (middle one) to make minor adjustments to the gray point.

In our example we're going to make a global (all channels) gray point correction using the Set Gray Point eyedropper . This will remove some of the blue cast in the shadows. For that we need to find an area that we assume should be 50% gray (a medium gray) and sample it with the Gray Point eyedropper. Make sure before you do this that the channel box is set to RGB, since we want to make a global adjustment, affecting all 3 channels at once:

In my example I decided to sample the following medium gray area in the image:

You probably won't see a lot of changes in the image when you do that (it only means that our previous corrections were pretty good), but every little thing helps. If you see a huge color shift, then try to sample again; the shift should only be minor. Remember that we set the Sample Size of the eyedropper tool to 5 x 5 average earlier? That's to avoid this huge color shift.

After this you will still see some areas in the image with a slight blue cast, like parts of the river bed or roads that are covered by shadows. Personally I don't think it's too shocking. It's also very natural to see this and it's up to you if want to adjust those areas or not. If you want, then continue with step 3.

Add a new adjustment layer by clicking on the Create a new fill or adjustment layer icon in the layers palette. Select in the menu that pops up Hue/Saturation...

In the Edit box select Blues (A):

The Eyedropper Tool (B) is selected by default, which means we're ready to sample an area that needs an adjustment.

Sample an area that has the blue cast. I went for an area in the upper right corner of the shot, the middle of the river bed beside those two trees:

After you've done that enter a value of -50 for Saturation (C) and click OK. Why not a value of -100? Personally I don't think it's a good idea to remove something completely that was originally visible with the naked eye. The blue cast is not a mistake, it's a reflection of the blue sky above. By removing it completely you tend to make the image look unnatural. We've already done that by using a small value like -50, but I think it makes the image look better, it takes away some of the distractions. So the value of -50 should be seen more as a suggestion than a rule.

At this point I decided to go bring down the green a little bit. Thankfully we made use of adjustment layers (a great advantage!) so we can back any time to make these kind of adjustments.
Double click on the Levels icon in the layers palette...

...and change the value for the Green channel from 52 to 64. Don't click on OK yet, but check and uncheck the preview box a few times to notice the difference; by reducing the green you will also bring in more magenta and this in return will increase the contrast between the green and brownish foliage, which in return makes the image more pleasant to look at. If you are using a value higher than 64, then you're going to lose too many details in the shadows, so keep that in mind.
After you're done previewing the result, click OK.

The last thing you can do now is sharpening the result (not too much).

This is the final result:

I hope you enjoyed this tutorial.

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