I am using the Nano text editor.
Before, there was color on every command I typed, but now I don't know what buttons I pressed as there are no more colors.
How to bring back on the syntax coloring?
You probably pressed Meta+Y, that Meta usually is Right Alt, so Right Alt+Y.
Press it again to undo the change.
You can also force colors temporarily with; taken from the
-Y <name> --syntax=<name> Syntax definition to use for coloring
nano -Y sh assign
Also, implicitly, the file must be marked executable in order for the syntax coloring to come up automatically, so just a quick check:
ls -l assign