I need a program/script that will press a specific key, for example, 'F3,' when a certain value changes or the color of a pixel changes in a designated area on the screen. Is there anyone who can write such a program or script? Of course, I am willing to pay for it.
Ofc need to be on MacOS.
In my experience, paying will hardly be necessary. There are very capable people here who are happy to help. But you will have to explain in more detail what you need.
I have a 2D game with two colored bars and a number on the screen. I need a script that, upon a change in, for example, the color or number, will press a designated key. In other words, the program must check the numerical value at a specific screen location and respond to changes by pressing a key. I am willing to pay for this service as I value when somebody want to help. 
Hello Kamil, and welcome to the forum!
In the Keyboard Maestro vernacular: a Trigger defines what event causes a Macro to execute its defined sequence of Actions.
In your case, the change in state of the image on the screen cannot be a trigger. You could, however, trigger the macro manually (e.g., with the Hot Key trigger) or automatically (e.g., with a Periodic trigger).
Once triggered, the macro could use a Pause Until action combined with something like a Pixel condition or a Found Image condition to detect the bar change. You can then respond to the change with the Keystroke action.
You may also consider some more advanced control flow mechanisms such as If/Then/Else branches and While loops.
Once you get started, let us know if you run into any issues.