In any version of Photoshop or Elements you have access to layer modes with the default being Normal. The mode is in the drop down box at the top left of the layers panel next to the layer opacity. 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. In this section we will describe the two members of the Darken group of blend modes used most by photographers - Multiply and Darken.
The most common of the Darken modes is the Multiply mode which mathematically multiplies the values in the current layer by those in the layers below. A typical use for this is to create an overall darkening effect to the image. This is then controlled with the use of the layer mask to render the effect where it is desired, and lowering the layer opacity. A vignette may be a typical effect, darkening the edges and corners of the image. The visual effect is much like that achieved by overlaying two slides in that the values of the two images reduce the transmitted light. The downside of Multiply mode for vignettes is an increase in saturation of the dominant color. In subtle vignettes this may be of little concern.
The typical use of Multiply mode is to darken an image with an adjustment layer. Adding any adjustment layer in Multiply mode Photoshop assumes you intend to use the content of the layer below as if it were the content of the adjustment layer and darkens the image in the same way as a duplicate layer in Multiply mode. This behavior is valuable in that the use of an adjustment layer without pixel content helps to keep the size of the file to a minimum and still get the desired effect. Modification can be made to the adjustment layer, but even without them the effect is applied.
In the first image we see the initial content of the image, and if you mouse over the image you see the effect of adding a Levels adjustment layer in Multiply mode. Every value in the image is darkened. This is not the effect we want so we need to mask off areas we do not want darkened in order to control the effect.
In the second image the brightest values of the clouds in the upper left were selected and a gradient was added to the mask to allow the Multiply mode to darken the sky values in a more controlled fashion. The opacity of the Levels layer was reduced to blend the darkening effect to the desired value. This is a very simple image modification and often is the easiest method of simply darkening a sky, or adding a darkening vignette to an image. In addition to the blend mode itself, other modifications can be made to the adjustment layer in order to further modify the effect. Changes to the values in a Levels or Curves layer can be used to make more subtle adjustments to the layer including modifying color. A Hue and Saturation adjustment layer in Multiply mode will look just like a Levels adjustment layer with no modifications, but adding an actual hue shift or saturation adjustment can help make subtle modifications that can be very valuable. There are no "rules" to these techniques, you simply need to play with the various options afforded by your choice of adjustment layer to help you control your image.
Another useful member of the Darkening Group is the Darken mode. In the Darken mode the content of the layer is only applied to the image below if the content of the layer is darker than the existing content. Anything lighter in the layer is ignored. The Darken mode can be useful where an image has been processed to two different values and composited to introduce the darker processing into the lighter image. Salvaging highlight detail in light sources, or near windows would be an example of this approach. Again, masking would also be a part of controlling the effect, but the Darken blend mode would keep lighter areas of the image from having an influence, especially if the contrast of the second image were higher than the background image.
One very useful technique is the application of a sharpening layer in Darken mode. In Unsharp masking the sharpening effect is created by brightening and darkening opposite sides of the edges within the image. In Darken mode the lighter highlights or glow of the sharpening are suppressed leaving only the darkening halos intact. This can be used to more aggressively sharpen details where the effect of the lighter halos is disruptive. Darker haloes tend to be less obvious. The third image shows a detail portion of an image sharpened rather aggressively with unsharp mask in order to show obvious halos. The sharpening was applied with the layer in Darken mode and if you mouse over the image you will see the lighter haloes appear around the edges of the subject.
In this instance the darker halos would obscure some of the tree limb detail and the sharpening would not be applied at this high a radius level to avoid that. This is simply to show the effect of the darken mode in suppressing the lighter values.
The effects of the Darken blend mode are applied channel by channel in an attempt to minimize harsh edge transitions that can occur if the effect were simply applied as a composite. The Darker Color mode that is in the Darken group actually applies the effect as a composite if you wish to compare the result. The Darken mode is therefore the more desirable mode as it introduces fewer opportunities for edge effects (artifacts) as part of the process.
There are other modes in the Darkening group that are generally less often used by photographers. Color Burn is darker than Multiply and tends to increase color saturation, especially in the mid-tones. Linear Burn is also darker than Multiply without the saturation increase.