Make sure the Always show all icons and notifications on the taskbar is unchecked at the bottom of the Notification Area Icon panel. If it is checked, it will grey everything out.
If the above didn't fix it then check under the Start Menu and Taskbar policy path for Hide the notification area, Prevent changes to Taskbar and Start Menu Settings, and Lock all taskbar settings policy settings. Play with this settings. Remember to set it to not configured if these settings fix the issue. This will prevent future issues related to group policies.
Also, your user could a member of another group, besides the administrators group, that might have same policy enabled*. If that isn't then maybe there is a 3rd party application that is installed and prevent you from change it. There is an example of this at this post.
Information About the Group Policies Refer to Above
More about the Hide the notification area group policy...
This setting affects the notification area (previously called the "system tray") on the taskbar.
Description: The notification area is located at the far right end of the task bar and includes the icons for current notifications and the system clock.
If this setting is enabled, the users entire notification area, including the notification icons, is hidden. The taskbar displays only the Start button, taskbar buttons, custom toolbars (if any), and the system clock.
If this setting is disabled or is not configured, the notification area is shown in the user's taskbar.
Note: Enabling this setting overrides the "Turn off notification area cleanup" setting, because if the notification area is hidden, there is no need to clean up the icons.
More about the Prevent changes to Taskbar and Start Menu Settings group policy...
Removes the Taskbar and Start Menu item from Settings on the Start menu. This setting also prevents the user from opening the Taskbar Properties dialog box.
If the user right-clicks the taskbar and then clicks Properties, a message appears explaining that a setting prevents the action.
More about the Lock all taskbar settings group policy...
Prevents the user from making any changes to the taskbar settings through the Taskbar Properties dialog.
If you enable this setting the user cannot access the taskbar control panel. The user is also unable to resize, move or rearrange toolbars on their taskbar.
If you disable or do not configure this setting the user will be able to set any taskbar setting that is not disallowed by another policy setting.