[If you want to change only particular folder icons or folder label colors see Color Folder Icons and Set Folder Colors via Context Menu.]
Directory Opus gets the default folder icon by asking Windows via a standard API.
Directory Opus 22.214.171.124 and later:
Directory Opus 126.96.36.199 and above should automatically pick up icon changes made by most desktop-customisation programs.
You can also change the icons manually, without any additional tools, via two simple registry settings. These registry settings will affect Directory Opus 9 and other programs such as Explorer.
Open the registry editor (RegEdit.exe) and go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer. Below this there may already be a Shell Icons key but if not create one; either way, go inside of the Shell Icons key and then create two String values with names 3 and 4. 3 should point to the .ico file to be used for Closed Folders while 4 should point to the .ico file to be used for Open Folders. See the screenshot below.
(The screenshot shows that RegEdit has also taken on the new folder icons. That only happens with the Vista version of RegEdit. If you're using an earlier version of Windows then don't worry if RegEdit continues to use the default yellow folder icons.)
Newer versions of Windows seem to pick up the change automatically. On the other hand, if you are using Windows XP then you will probably need to force Windows to rebuild its icon cache before the new folder icons will appear. You can do this using Microsoft's TweakUI tool: Load TweakUI, select Repair in the tree, ensure Rebuild Icons is selected in the drop-down and then click Repair Now. After a few seconds you should see the icons in Opus change. There is no need to reboot.
Older versions of Opus:
There are several icon-changing programs for Windows which let you choose a different folder icon in Explorer. Unfortunately, most of these programs only affect Explorer and have no effect on Opus or any other application, unless something special is done either in the icon changer or in the application.
Either the icon changer has to explicitly target the application (e.g. dopus.exe) or the application has to have some special code which picks up the changes applied to Explorer. Opus has this special code as of version 188.8.131.52 but if you are using an older version of Opus then you will need an icon changing program which can do the work on its end.
Several forum members have reported success using IconPackager. This is an icon-changing tool which allows you to specify the programs that it affects. You have to tell it to target dopus.exe as well as explorer.exe.
There is some information in these two threads: