Custom Drop Down Colors

New to creating a script in PDF, so I need some help.  I need to the drop down color to be red, yellow, greem or trasparent based on a selection "high" - red, "Medium" - yellow, "Low" - green, and "N/A" to be no color. 

I read some similar posts and created the script below. I think my problem is how I am loading it under "actions". Can someone please validate the script then walk me through the process to load the script? Thank you! 

if (event.value == "Low") event.target.fillColor = color.green;

else if (event.value == "Medium")event.target.fillColor = color.yellow;

else if (event.value = "High")event.target.fillColor = color.red;

else if (event.value =="" || event.value == " ") event.target.fillColor = color.transparent