How to set background color for grayscale image

[harryspier]

Using MagickWand API I want to set the background color to white for a grayscale image.

I have an example for doing this in rgb as follows but I'm not sure how to specify "white for a grayscale image in the following example:
pwo = NewPixelWand();
PixelSetColor(pwo,"rgb(60%,60%,60%)");
MagickSetImageBackgroundColor(mwf,pw0);

Thanks,
Harry Spier