For most items, the black color is just fine. But sometimes, it is more realistic to make your shadow the same hue of your background paper but darker.
After you see the difference, you will have a better understanding.
-Open a background paper of your choosing.
-Open the element you would like to drop shadow, and place it on top of your background. Add a standard "low" drop shadow. Name it Element 1.
-Drag another copy of the same element onto the paper next to the first element. Name it Element 2.
-Duplicate Element 2 and name it Shadow. Place the shadow layer below the Element 2 layer.
-Right-click on the shadow thumbnail and choose "Select Layer Transparency" or "Select Pixels". This will put "marching ants" around your element. What we want to do is "fill" this layer with a color that is the same shade of our background but darker.
-Edit>Fill Selection. A new dialog box will appear. Here, you want to select "Use Color" from the drop down menu, and using the mouse, move the cursor over the background until the Eyedropper Tool appears. Select the background color. Then go to the Color Picker and directly below the circle selector on the Color Picker, select a darker blue than the background color. Choose a color that would make a realistic shadow color.
-Click OK and Select>Deselect to get rid of the marching ants
-Look at your layers menu and you will see that shadow layer is now filled with the color you just selected. This is your new shadow!
-Next, we want to put the shadow in a realistic place. Select the shadow layer. Move this layer down two spaces (or pixels) and to the right two spaces (or pixels), using the arrow keys on your keyboard. This is your most natural size and placement of a shadow, but again, this is all personal preference.
-The shadow is in place, now all we need to do is soften it. First, let's put a little less Opacity in it. Making sure the shadow layer is active, go up to the Opacity slide bar and slide it left, to about 70%.
-Finally, we want to "blur" it up just a bit, so the shadow isn't so harsh. With the layer still active, go to Filter> Blur> Gaussian Blur. Choose a radius of about 7.8 to 8.8 pixels. Select "OK."
-You now have the new and improved drop shadow!
Special thanks to Theresa Kavouras