You'll need to use either a validate or format script on the field that is being colored. These events are activated whenever a field value is modified.
Something like this works fine
if(event.value == "Thing1")
event.target.fillColor = color.yellow;
else if(event.value == "Thing2")
event.target.fillColor = color.blue;
Here'a an article on the topic that also includes a similar script:
Finally there is a possible impediment to this scheme. If Acrobat/Reader is setup to highlight form fields, then the color changes may not be seen. You might want to add this code to a document script to turn off highlighting when the document is opened.
app.runtimeHighlight = false;