How to add a fill color to my scripted form field?

I would like to add the color white to the IRdate field created below.  The reason is to cover up any previous text under the field.  At the moment, when I place the IRdate appearance the old PDF text comes through.  I know I can manually change it so there must be a script way of doing this.  Any ideas out there?  Thanks!

//Establish Scaling
var inch = 72;

//Position Rectangle for RFC Stamp Field
var aRect = this.getPageBox( {nPage: 0} );
aRect[0] += 16.25*inch;  // from upper left hand corner of page.
aRect[1] -= 0.509*inch;
aRect[2] = aRect[0]+0.480*inch;
aRect[3] = aRect[1]-0.115*inch;

// Construct RFC Stamp Field
var field_name = "IRdate"
var f = this.addField(field_name, "signature", 0, aRect)

sig_field = "IRdate"

var f = getField(sig_field);

// Choose handler
var ppklite = security.getHandler("Adobe.PPKLite");

// Login -- *****change as appropriate*******
ppklite.login("Password-something", "/C/Documents and Settings/Zap/My Documents/Digital_ID/Zap.pfx")
f.signatureSign(ppklite,{password: "Password-something", appearance: "IRdate"});
ppklite.logout();