Mi is not technically open. When you vote in the primary you have to ask for an R ballot or a D ballot.
Michigan
Primary Election Process
Michigan is an “open primary” state. You can choose the party’s ballot you wish to vote. This decision does not register you with that party and it is a private decision.
If you have any questions about your state’s primary election, please contact your local election officials.
The fact that they have separate ballots for the two parties does not make it a closed primary. They ask to know which ballot to provide to the person voting.
That means open.
Closed means you have already been registered and listed as being in a particular party.