You can use something like this as the custom Validation script of the field:
if (event.value=="Item1") event.target.fillColor = color.red;
if (event.value=="Item2") event.target.fillColor = color.blue;
if (event.value=="Item3") event.target.fillColor = color.green;
Make sure you set the option to commit the selected value immediately under the field's Properties.
Thank you for your help Gilad. The code works, but some of the colours don't work (yellow, magenta, orange). Any suggestions?
Read here about how to use the color object correctly: