Here's how I achieved this, has a dependency on jQuery. Register the function on the form load event.
function ColourPriority() {
$("#new_priority").children().each(function () {
var a = this;
// P1
$(a).val() == 100000000 && $(a).css({
background: "#F6CECE"
});
// P2
$(a).val() == 100000001 && $(a).css({
background: "#F8E0E0"
});
// P3
$(a).val() == 100000002 && $(a).css({
background: "#F3E2A9"
});
// P4
$(a).val() == 100000003 && $(a).css({
background: "#F7F8E0"
});
// P5
$(a).val() == 100000004 && $(a).css({
background: "#D0F5A9"
});
// P6
$(a).val() == 100000005 && $(a).css({
background: "#E0F2F7"
});
});
}
..and this is how it looks. The background color persists once the field is unselected.