Change Color of Text Field from Drop Down Menu Selection

Hello,

I am and in need the same function. The field is called "Ankle DFR" and the selection options are (no commas): Limited, WRL, and Hypermobile. I would like it to turn yellow when I select "Limited" and "Hypermobile". 

So I created a drop down menu, opened properities, clicked the calculate tab, and entered the following in the Custom Calculation section:

event.target.fillColor = (this.getField("Ankle DFR").valueAsString=="Limited") ? color.yellow : color.transparent;

However, it only goes yellow when I click once and see the list of options. But when I select the desired choice, Limited, it goes back to its original color. I would like it to remain yellow (or whatever color), when I select anything other than WRL. Thanks in advance for any help!