Mac laptop doesn't display on the screen/TV
Symptoms
When connecting a MacBook to the external HDMI cable on the screen, the screen flickers and displays a message about adjusting the resolution and then goes blank.
This seems to happen mainly on Macs when using an adapter to get the HDMI connection as opposed to Macs that have built-in full-sized HDMI ports.
Cause
The TV is using the colour and resolution settings from the previously connected device that may be not be supported by the current HDMI connection. This is especially true when using an adapter to make the HDMI connection.
Solution
Use the Settings on the TV remote and go to the "External Device Manager" and select the "HDMI UHD Color" option
Then, make sure the current connection (HDMI3 in most rooms) is not selected (not a green colour) - or to be safe just unselect them all (see below)
From what we can tell, enabling of HDMI UHD Color option occurs automatically in the background when the connected device supports it. However, it doesn't seem to disable the option when the connected device/adapted doesn't support it. So, if the last device connected enabled the option and the next device/adapter doesn't support it, the only way to get it to work is to use the TV remote to disable it.