Saturation is one of those things that we tend to either ignore or overdo. In some color images, it's the actual colors that account for the interesting-factor — and in those cases, saturation processing is very important.
I have the sneaking suspicion that most of us (yes, I'm at fault too) will use an RGB saturation adjustment layer (or an ACR adjustment) to boost the color saturation. This is probably OK for very small adjustments on the order of less than 5 or 10%. Once you go beyond that, you'll start clipping the color highlights and end up losing tone details and textures.
But have no fear, there is a better way! The LAB (Lightness, A, B) color mode is quite useful for making higher quality adjustments to the saturation. I'm guessing that most will avoid LAB editing because it seems less intuitive, or because there's no immediate need to use it. In reality, it's no more difficult to use than the RGB color mode. So if you have an image that's begging to be saturated, try out this technique and I'm certain you'll find just how powerful it can be.
I've put together a Photoshop action that might help speed up the whole process. Laziness is usually why I'll opt for the RGB saturation method vs the LAB method, so this action should make the process less tedious. Just apply any touch-ups or adjustment layers to your original image like you normally would, select the top layer when you're ready to bump your saturation, and run the action. This action basically takes care of everything, but it gives you the ability to adjust the LAB curves half way through the execution. After getting the LAB curves adjusted (as outlined in step 4 below), you'll be presented with a “Duplicate Layer” dialog — You MUST select your original document from the second drop down menu for things to work correctly. Give it a try, and let me know if there are any problems with it. Also, before you run it, you might want to run through the steps below just to get a better idea of what it's doing.
1. OPEN YOUR IMAGE IN RGB MODE
The RGB color mode is the most common to work with, so I'll assume that most of us use this as our default. We can still work in RGB even though we're doing adjustments in LAB color mode — it's just a little extra work. So open up your image as you normally would and apply any touch-ups such as spot removal or any other cloning or cropping. Once you're comfortable with the base image, we'll start looking at the color and contrast.
You can also go a different route than what I'm showing here — you could actually start up in LAB color mode straight out of the RAW file (for those of us processing RAW), skip steps 2 and 3, go straight to boosting the saturation, and just convert back to RGB when you're done. What you want to avoid is excessively switching back and forth between RGB and LAB color modes.
2. RGB CURVE ADJUSTMENTS
After applying my touch-ups, a curves adjustment is usually the first thing I do. The RGB curve is great for increasing contrast using the traditional “S” curve as I've shown here. This adjustment also gives the appearance of increased saturation too, so it's a good idea to apply it and dial in your contrast prior to adding any color saturation.
3. OPEN A COPY IN LAB MODE
Make a copy of the visible image by doing a “Stamp Visible” command (Ctrl+Alt+Shift+E). This gives us a single layer that contains all of the underlying adjustments we've applied. Now take this layer and duplicate it to a new document. Once in that new document, change the color mode to “LAB”. Why not just change the color mode in the original file? You'll lose any RGB-specific adjustment layers such as the curves adjustment we just applied (go ahead, try it and see what happens). OK, so now we're working with LAB and we're ready to boost the contrast.
4. LAB CURVE ADJUSTMENTS
We'll do the saturation adjustment with a curves adjustment layer, but this time we'll be adjusting each channel individually. Start with the “A” and “B” channels and make the adjustments as shown in the screenshot. Be sure to pull each side of the curve in equal amounts, otherwise you'll end up with some funky color shifts. Same goes for adjustments between the two channels — try to keep them in the same ballpark. The last step is to go back to the “Lightness” channel and put a slight “S” curve into it to fine tune the contrast after adjusting the saturation. Again, extra contrast adds to the appearance of extra saturation.
5. COPY BACK TO THE RGB IMAGE
Once you're happy with your saturation adjustment, you can merge the layers, stamp visible, flatten, etc. to give you a single layer with all of the color information. Now copy that layer back to your original image — it should convert itself back into RGB when you copy it over.
Once you're back in the original image, you're pretty much done with the adjustment. You can now toy around with masking and opacity settings to get the final look you're after. If you're seeing localized spots that seem too saturated our out of place, you can just brush over them on the mask with a low opacity brush until you get the results you need.
A COMPARISON OF THE RESULTS
Now for the fun part — let's evaluate just how much better this technique is from a standard saturation adjustment that most of us are familiar with. Sometimes the old RGB method will work out better for you, but in my opinion, the LAB saturation is much more natural with deeper colors and less highlight clipping.