exhaust manifold turns cherry red?

Hello,

An exhaust manifold can turn red, if there is a restriction or clog in the system. It can be inside the manifold or rights after it. As the hot exhaust gases are not able to escape properly, it builds up inside the manifold and overheats metal.
It can be due to any type of restriction in the exhaust tubing or catalytic converter.

You need to to a through inspection of the exhaust piping and converters to see if there is a clog.

https://www.2carpros.com/articles/how-exhaust-systems-work

Try starting the car and wait for the exhaust to get hot then use a heat gun to see where the temperature drops of from hot to relatively cooler as there is a chance the restriction is right before the cooler part.

Please let me know of any questions.
Thank you.