There are several ways you can move categories between mailboxes. The easiest, via an appointment form, does not maintain the category colors. Outlook will assign the colors to each category. If you don't care about the color, this is not an issue. If you only have a few categories where the color matters, it's simple enough to change the colors yourself.

1. Add all categories to an appointment, copy it to the calendar in the new mailbox, run the command to Upgrade to color categories.
2. Use a macro to export a list of categories and a macro to import the list.
3. Use MFCMAPI to export the categories.
4. Use Powershell to export and import.

These scripts work with Outlook on your Windows Desktop. To use them, open the PowerShell ISE, paste the script (and edit) then Run. Tip: You can use the "regular" Powershell window, pasting the entire script in at once, but it's easier to edit the file path in the ISE editor.

1. On the Windows Start menu, start typing powershell.
2. When it comes up, click on Windows PowerShell ISE to open it.
   
3. Paste the desired script in the editor.
4. Change the filepath and name as needed. You can save it as a CSV or txt file. Use double quotes around the name if it contains spaces.
5. Click Run Script or F5 to run the script.
   

For these screenshots, I exported my categories then used the delete script to clear them before re-importing.

Export Categories

These scripts create a simple comma-separated list containing the category name, color code, and shortcut. You can edit the list in NotePad before importing it into Outlook.

"Name","Color","ShortcutKey"
"Internet Orders","23","0"
"Newsletters","3","0"
"Outlook MVP","8","0"
"MVP","2","0"
"GTM","17","0"

Use this script to export the categories in the default account's mailbox.

$outlook = New-Object -ComObject Outlook.Application
$namespace = $outlook.GetNamespace("MAPI")
$namespace.categories | Select-Object -Property Name, Color, ShortCutKey | Export-Csv -Path D:\Categories.txt -NoTypeInformation -encoding utf8

Use this script if you have more than one account in your profile and want to export the categories from the non-default account. The folder picker dialog comes up - select the account name that you want to export.

$outlook = New-Object -ComObject Outlook.Application
$namespace = $outlook.GetNamespace("MAPI")
$namespace.PickFolder().Store.Categories | Select-Object -Property Name, Color, ShortCutKey | Export-Csv -Path D:\Categories.txt -NoTypeInformation -encoding utf8

Import Categories

These scripts import the categories from a text file. Note that if the category name exists, the script will return an error on that entry in the PowerShell window but will add any category that does not exist. If you want to change the category colors or shortcuts, you'll need to delete the categories first as the script will not update existing categories.

Use this script to Import categories into the default mailbox.

$outlook = New-Object -ComObject Outlook.Application
$namespace = $outlook.GetNamespace("MAPI")
Import-CSV "D:\Categories.txt" | 
ForEach-Object { 
    $namespace.Categories.Add($_.Name,$_.Color,$_.ShortCutKey) 
}

$outlook = New-Object -ComObject Outlook.Application
$namespace = $outlook.GetNamespace("MAPI")
$sourceStore = $namespace.PickFolder().store
Import-CSV "D:\Categories.txt" | 
ForEach-Object { 
   $sourceStore.Categories.Add($_.Name,$_.Color,$_.ShortCutKey)
}

Delete Categories

To delete categories from the list, first export the list then run it to remove all categories from the list. Because this script moves forward, you may need to run it a few times to remove all categories. If the list is in alphabetical order, fewer passes may be needed.

$outlook = New-Object -ComObject Outlook.Application
$namespace = $outlook.GetNamespace("MAPI")
Import-CSV "D:\Categories.txt" | 
ForEach-Object { 
    $namespace.Categories.Remove($_.Name)
}

$outlook = New-Object -ComObject Outlook.Application
$source = $namespace.stores | 
Where-Object { $_.displayname –eq ‘alias@domain.com’}

Import-CSV "D:\Categories.txt" | 
ForEach-Object { 
  $source.Categories.Remove($_.Name)
}

Outlook Color Category names and index number

Beginning with Outlook 2019/365, Microsoft updated some of the category colors and their names.