You can change the colour of the field with this:
this.resolveNode("NumericField1").fontColor = "255, 0, 0";
But it will change the caption colour as well.
Change just the value with this:
if (xfa.host.version < 8) {
this.resolveNode("NumericField1").fontColor = "255, 0, 0";
}
else {
var CaptionColorBackup = this.resolveNode("NumericField1").caption.font.fill.color.value;
this.resolveNode("NumericField1").font.fill.color.value = "255, 0, 0";
this.resolveNode("NumericField1").caption.font.fill.color.value = CaptionColorBackup;
}
I tested this in ES2 just using a click event to change the field colours.