In any version of Photoshop or Elements you have access to layer 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 can be made more productive by using a shortcut. 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. Without other modifications to adjustment layers or differing content in the layers many of the modes will appear to have no effect.
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 Darkening group all serve to darken the image in various ways depending on the actual mode and layer content. Whites in Darkening modes 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 Comparitive group reveals differences between layers and is useful for aligning images in composites. The Component Color group decides how a layer is applied to exisitng 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 and explore them more deeply elsewhere.
Luminosity and Color
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.
The first image is a sunset with some modifications to each of the color channels in Levels to lighten or darken certain tones. This was done in Luminosity mode so the changes would not affect the colors in the image. Roll your mouse over the image to see the result with the Levels layer in the default Normal blend mode. This would not be useful as it shifts the colors in a bad way. When using the Luminosity mode the changes influence the contrast or brightness of the image without shifting the colors.
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 are 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 we see the control that the Color blend mode affords us. In this case a correction was made as a hue and saturation adjustment to add magenta interest and saturation to the reflections of the sky in the surface of the pond. Existing color was there, but muted, so the desire was to enhance the color values. Doing so darkened the image, so the layer mode was changed to Color. This way the modifications applied to the hue and saturation to enhance the color are not applied to the image luminosity. Roll over the image to see the layer applied in the Normal blend mode. In this case the effect is subtle. Which effect you prefer depends on the blend mode you apply to the layer.
The explanation for the ability to control luminosity separately from color is simple if you remember 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, and knowing that you can make your editor ignore the color or luminosity with blend modes is the important part of the conversation.
To try the technique, if you are not familiar with layer modes, add an adjustment layer and click OK without making any corrections. In the upper left corner of the Layers dialog there is a drop down box that now says "Normal". Click on the arrow to the right and choose Luminosity to make changes to density or Color to adjust color without affecting the other. Both choices live at the bottom of a stack of blending choices, most of which you will not have much use for. Now select individual color channels and play around to see what you can do. Depending on the content of your image you will see some areas change more than others. The Luminosity mode is where you can be the most creative.
In Photoshop the primary control method to use is Curves as any value in the image can be use as a control point. 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.
There are quite a few other blending modes as you can see on the drop down list. A few others are very useful for photography and will be explored in another section. In the meantime you can get used to using these two very useful blending modes to give you more control over how you edit your files.