In any version of Photoshop or Elements you have access to layer Blend Modes with the default being Normal. If we make a color or density correction to an image, the default Normal mode of the layer means we influence both the luminosity and the color of the image. That is a default we can override. There are quite a few layer Blend Modes available to us and each does something different to the image depending on the content of the layer and its relationship to the layers below it. We will start with using just the two modes that allow us to separate the image color and content.
Cycling through the Blend Modes in Photoshop is now automatic as you move your mouse down the list in the drop down box. As you pass over each blend option the effect is shown on the image. This makes it possible for you to easily see the effect immediately. In older versions with the Move Tool (V) active you can cycle through the Blend Modes using the Shift key and the + and - keys or the up and down arrow keys to move forward or backward through the modes.
There are six groups of Blend Modes including the Normal, Darkening, Lightening, Contrast, Comparative, and Component Color groups. The Normal group modes have no immediate effect on the image except what the layer content provides. The normal mode just applies the layer content without changes. The Dissolve mode does nothing at 100%, but as the layer opacity is reduced a noise pattern is introduced. In my estimation there are better, more controllable ways to introduce noise into an image if needed. Dissolve mode has little value to a photographer.
The Darkening group modes all serve to darken the image in various ways depending on the actual mode and layer content. Whites in Darkening modes simply disappear. Similarly the Lightening group lightens the image and blacks disappear which is useful in compositing. The Contrast group lightens values above middle gray and darkens values below. The Comparative group reveals differences between layers and is useful for aligning images in composites,. The Component Color group decides how a layer is applied to existing layers relative to hue, saturation, color or luminosity.
This series is very basic and describes the Blend Modes most often used by photographers. I encourage you to explore their effects and usefulness. I will primarily discuss those blend modes of most value to photographers for normal photography processing.
By changing the mode of the layers to either Luminosity or Color we can restrict the corrections to where we want them to be. That allows us to make color corrections to an image without changing the density of a particular area, or modify brightness or contrast without influencing the color.In the first image at right changes were made to the sky using Levels. Normally you can simply darken the image in Normal mode with no problem. Using individual channels gives you more control over these changes, but modifications to individual channels also changes the color of the image. Putting the Levels layer in Luminosity mode allows the changes to the individual channels to modify the sky densities without the color shift. Mouse over the image to see the effect of the Levels changes in the Normal mode. Modifications to the blue channel shifts the colors toward yellow in the Normal mode, but with the Levels layer in Luminosity mode only the density changes.
A density change in anything but neutral tones will always primarily modify the predominant color of that area compared to the others and introduce a color shift. This is usually ignored by most Photoshop users as it is minor in most cases. Minor or not, color shifts are usually a bad thing and fortunately are easy to avoid. Using individual channels while in Luminosity mode gives you the kind of control over the density of colors in the image that you had with filters in black and white film. Elements users can use the technique in Levels and Hue and Saturation adjustment layers to modify color densities. Photoshop users can add Curves, Black and White, and Selective Color adjustment layers to their tool chest. Other adjustment layers may also make minor modifications but may be less effective. The bottom line is that changes to an image that primarily need to influence brightness and contrast are best accomplished in Luminosity mode.
In the second image a Hue and Saturation layer was used in Colorize mode to change the eye color. In Color Mode the changes show by modifying the image color without having an affect on the image densities. Roll over the image to see the increased density of the blue color in Normal mode. Color mode allows you to influence color while retaining the original image luminosity. Brightening the eyes in the adjustment layer can introduce shifts in saturation as lightening or darkening a color simply introduces white or black. The differences are often subtle, but important.
The explanation for the ability to control luminosity separately from color is that Photoshop works in the L*a*b mode in the background. The L channel is luminosity, and the *a and *b channels are the green/magenta (tint) and yellow/blue (temperature) color channels respectively. While that is interesting to know, in almost all cases you will be working in RGB and not L*a*b. Using Blend Modes you can make your editor ignore the color or luminosity as needed to effect the image without resorting to using the less comfortable L*a*b mode.
A favorite of mine is to use the Black and White adjustment layer in Photoshop in Luminosity mode. Doing so allows you to remap (modify) the densities of the colors in the image rather than convert the image into Black and White. The Black and White adjustment layer has no affect in Color mode as there is no color to modify.
In Photoshop the primary control method to use is Curves as any value in the image can be use as a control point in any channel. Elements users do not have quite as much control as Levels restricts you to center point control, but even so, there are the ends and the output sliders as well to play with. Some changes will bring dramatic changes and others will be more subtle. This will be on an image by image basis as the amount of a particular color channel controlling various parts of the image will vary depending on how far from neutral the color is.