Dimming inactive Dock icons
I don't know how to dim inactive icons in the Dock. However, some people prefer the old-style two-dimensional dock that you can activate. It's certainly easier to discern which apps are open on this view.

To activate the 2D dock, open Terminal. Then type in the following command:

defaults write com.apple.dock no-glass -boolean YES


From there, you can log off &amp; back on, and your Dock will have changed. If you ever need to revert this setting back run the above command again, but change the word YES for NO.