To show an example of what can be done, your html code for a pop-up could be like this:
<font color="{expression/expr0}"><b>{GRAVEDAD}</b></font>
In this case my field is called "GRAVEDAD" with three possible values: SOLO DAÑOS (only material damage), HERIDOS (injuries), MUERTO (diseased). If you would like to vary the color of the text, you can use an Arcade expression inside the color tag (switch to HTML view to reveal the html code).
The expression in my case would be:
var dct_grav = {"SOLO DAÑOS": "#ff8c00", "HERIDO": "#ff0000", "MUERTO": "#800080"};
var gravedad = $feature.GRAVEDAD;
var color = dct_grav[gravedad];
Console(color);
return color
This will return the color code (purple, red or orange) depending the field value GRAVEDAD, resulting in: