This code will change the field's fill color. If you want to change the text color as well you'll need to provide more details:
var v = Number(event.value);
if (v>=0 && v<=26) {event.target.fillColor = color.green;}
else if (v>26 && v<=50) {event.target.fillColor = color.yellow;}
else if (v>50 && v<=60) {event.target.fillColor = color.red;}
else event.target.fillColor = color.white;
It should be placed as the field's custom Validation script, by the way.
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