The microsoft docs here (https://docs.microsoft.com/en-us/azure/azure-resource-manager/resource-group-using-tags) indicate that the colon (:) character is valid tag name character.
The CLI and Java SDK both allow tagging resources containing a colon in the tag key.
In the Azure Portal, I get an warning when attempting to add/edit a tag key with the colon character.
I'm assuming this is a Portal Azure bug?
Github issue: https://github.com/MicrosoftDocs/azure-docs/issues/11340
Note: I can't seem to add links and images because my "account is not verified"