Colouring the signature field after signing

Hi,
I would like to change the colour of the signature field after it has been signed. I am using the code as below, but the signature field still remains white.

function colorMerytorysta1DarkGreen() {

var merytorystaField = this.getField("Merytorysta1");

if (merytorystaField.signatureInfo.numSignatures > 0) { 

merytorystaField.fillColor = color.red;

} else {

merytorystaField.textColor = color.transparent;

}

}

colorMerytorysta1Red();

I also tried changing the condition to an empty value, but still no result