For starters, where does this script go?
var cat = this.getField("Combo Box 1");
var color = this.getField("Text Field 1");
color = "Orange";
color = "Black";
case "Mister Gray":
color = "Gray";
color = "Must be a dog";
The answer to the question "where does the script go?" depends on the answer to the question "what should the script do?".
In this case it seems you're trying to assign a value to the color field based on the value of the cat field.
What you can do in that case is place the code as the custom calculation script of the former, and change the first two lines to:
var cat = this.getField("Combo Box 1").value;
var color = "";
And at the end of the code this line:
event.value = color;
Seems to work. Thanks!