canvas size with backgroundcolor

Hi,

this part of my script defines and set a new background color

there-after the canvas size of image is extended

var warmgrey = new SolidColor();

warmgrey.rgb.red = 251;

warmgrey.rgb.green = 247;

warmgrey.rgb.blue = 248;

app.backgroundColor = warmgrey;

docRef.resizeCanvas(docRef.width , docRef.height + missingtop +2  , AnchorPosition.BOTTOMCENTER)

but how can i set that the backgroundcolor is used for resizeCanvas, instead of white or other options in de canvas pulldown options.

thanks,

Johan