It doesn't seem to be possible on Preview as is. However there is a way around which isn't too complicated (atleast for me). Once you've signed in the colour of your preference and it looks good on the screen, do the following:
1. On the top menu, go to File>Take Screen Shot>From Selection. It will prompt you to make a selection.
2. At this point, select the entire page of your document. Make sure to select the entire page to maintain the original page dimensions. If the original page is portrait, rotate it so you can see the entire page on your screen without minimising it too much. This will open a new file in preview with your screen shot selection.
3. Now save this new file. By default it saves it in png format, however you can select pdf as the format to save in. If you had to rotate the page as I have described in step 2, you can rotate it back to it's original layout
The quality of your signature will be exactly how you see it, if at the time of selecting the screenshot, your view is zoomed to 100% or close to it (that is why I suggest rotating portrait pages to landscape).
Hope this helps!