What replace serial port monitot color ttheme ?
Although the Arduino IDE does have a configurable theme, I don't think there's any easy way to modify the colors of Serial Monitor.
maybe it's time to think about it
If you use a telnet client instead of the serial monitor, telnet has a simple command set to control the terminal display.
While telnet is usually run over a network, it can be implemented over any serial transport layer.
Alternately, you could write some PC terminal code as part of your project that looks for keywords/tokens and sets the font colour as you like.
To simplify your starting point, there are many terminal protocols that support colour and other attributes. (ANSI, VT100 etc)
Serial alone is a generic connection method that has as little overhead as possible so that it ‘just works’ everywhere. How you implement your protocol is up to you.
Maybe a product opportunity for you to sell...?
Or an opportunity to earn the gratitude of many and give back to the Arduino project by making a valuable contribution to the Arduino IDE's codebase.