Yes, I meant change them completely to look different. The Apple icons are boring, especially for mail and safari and I often get them mixed up because they are both blue. I know one is round but still...I changed them in Mavericks and I enjoyed the view a lot more. I remember when we used to be able to change the skins on Macs which included the scroll bar to another color. I loved that. The fun stuff is gone for me.
Go to the apps folder, select the app, get info. Click on the icon. paste. (assuming you have the icon you want on the clipboard)
That should change it in Finder. Now drag it into the dock. If it's already in the dock you may need to remove and re-add it.
