.h264 videos green

Hello themanfromhere,

Welcome to Apple Support Communities. 

We understand that MP4.h264 videos show green on your Mac but play as expected on other devices. We are happy to help.

We’d like to have you complete some isolating troubleshooting steps—test in safe mode and new user account.

Here’s how to start in safe mode to test: Start up your Mac in safe mode - Apple Support

"Start up an Intel-based Mac in safe mode

1. On your Mac, choose Apple menu  > Shut Down.
2. After your Mac shuts down, wait 10 seconds.
3. Restart your Mac, then immediately press and hold the Shift key.
4. Release the Shift key when you see the login window."

Here’s how to add a new user account to test: Set up users, guests, and groups on Mac - Apple Support

"Add a user

1. On your Mac, choose Apple menu  > System Preferences, then click Users & Groups.
2. If the lock at the bottom left is locked , click it to unlock the preference pane.
3. Click the Add button  below the list of users.
4. Click the New Account pop-up menu, then choose a type of user.

• Administrator: An administrator can add and manage other users, install apps, and change settings. The new user you create when you first set up your Mac is an administrator. Your Mac can have multiple administrators. You can create new ones, and convert standard users to administrators. Don’t set up automatic login for an administrator. If you do, someone could simply restart your Mac and gain access with administrator privileges. To keep your Mac secure, don’t share administrator names and passwords.
• Standard: Standard users are set up by an administrator. Standard users can install apps and change their own settings, but can’t add other users or change other users’ settings.
• Sharing Only: Sharing-only users can access shared files remotely, but can’t log in to or change settings on the computer. To give the user permission to access your shared files or screen, you may need to change settings in the File Sharing, Screen Sharing, or Remote Management pane of Sharing preferences. See Set up file sharing and Share the screen of another Mac.

1. For more information about the options for each type of user, click the Help button in the lower-left corner of the dialog.
2. Enter a full name for the new user. An account name is generated automatically. To use a different account name, enter it now—you can’t change it later.
3. Enter a password for the user, then enter it again to verify. Enter a password hint to help the user remember their password.
4. Click Create User.
5. Depending on the type of user you create, you can also do any of the following:

• For an administrator, select “Allow user to administer this computer.”
• For an administrator, select “Allow user to reset a password using Apple ID.”
• Use Sharing preferences to specify whether the user can share your files and share your screen.

For information about Apple’s privacy policy, see the Apple Privacy Policy website.

If your Mac or Magic Keyboard has Touch ID, a new user can add a fingerprint after logging in to the Mac."

Let us know if you have further questions.

Take care.

Hello themanfromhere,

We apologize for the confusion. Please try safe mode first while still in your current user. Safe mode prevents certain software from opening and also forces the computer to run some disk checks similar to First Aid. This can actually resolve some issues. You may then want to try a new user if the issue persists in safe mode.

If you still have questions, reach out to Apple Support directly, as this may require closer guidance: Get Support

Cheers!