Saving a labeled image

I have used the function `bwlabel` to label an image as follows:

    [L, num] = bwlabel(I, 8);

For the the image I'm working with, say I have got `20` lables. Thus, when you run `unique(L)`, you will get a number list up to `20`.

The issue is that when I `imwrite(L)`, and the run `unique` on that new written image, I get the values of the pixels as their intensity in the original image I labeled, and not as a labeled image.

How can I `imwrite` that labeled image, while keeping it a labeled image?

Thanks.