How to handle text on bitmap during onPartialUpdate?

Hello, I'm a newbie and I'm experimenting with code to understand better CIQ sdk.

I'm trying to achieve a blinking ":" between minute and hour text with a background image but when I try to hide the white ":" something strange happens, I saw a white contour. I think I'm miss something. Every suggestion is appreciated.

thanks a lot.

the code is:

function onPartialUpdate(dc as Dc) as Void {
var clockTime = System.getClockTime();
if (sec==0) {
dc.setColor(0xFFFFFF, Graphics.COLOR_TRANSPARENT);
dc.drawText(dc.getWidth()/2+1 , dc.getWidth()/2+1, Graphics.FONT_SYSTEM_NUMBER_THAI_HOT, ":",Graphics.TEXT_JUSTIFY_CENTER | Graphics.TEXT_JUSTIFY_VCENTER);
sec++;
}
else {
sec=0;
dc.setColor(0x000000, Graphics.COLOR_TRANSPARENT);
dc.drawText(dc.getWidth()/2+1 , dc.getWidth()/2+1, Graphics.FONT_SYSTEM_NUMBER_THAI_HOT, ":",Graphics.TEXT_JUSTIFY_CENTER | Graphics.TEXT_JUSTIFY_VCENTER);

}
}
and the background image is create with the drawBitmap method.
see the image below.
I think