I've been watching Pete's Garage on YouTube and the guy is absolutely amazing and very enlightening on the subject. He has at least one video on engine painting but he mentions it in a number of other videos.
However, he doesn't really expound on why an engine should be painted. What are the benefits of painting an engine? For example, my Tacoma has an aluminum block and aluminum heads but they are not painted. Why are some other engines painted?
For most practical purposes, it's done for cosmetic reasons.
One might prefer to use black over silver for better heat dissipation but the motivation is almost always going to be to enhance appearance.
In cast iron blocks, the paint can act as a means of rust prevention.
It is mainly for aesthetics.
Something else it can do for you is to allow you to see when you have a leak. I've seen blocks painted white for this reason.
Something you didn't mention was whether you are talking about painting the outside or the inside of the engine. If done right, painting the inside of the engine can result in oil returning faster to the oil pan. I'm not sure that it is done that often, but it's another reason for painting it.
If one wants cosmetic engine, don't paint it have the outside chromed unless you desire to upgrade your car heater performance. For the Simple reason heat dissipation. Two Factor, Thermal Conductivity, and Thermal Emissivity. Best material to get heat from material to air etc, some material are better at it, call emissiviity. My engine was 25 year old, minimal rust, engine rebuilt they painted it, black, them up, no alarming and with in specs - if your car heater needs an upgrade - painting is an option. Otherwise, I would avoid, the hotter the engine the softer the material, the quicker the wear, I doubt if it is significant; but, it is there. Consult https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html https://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html
