Here is the original watercolor with a plain, no distraction, background.
My idea was to come up with a design that resembled the chocolate spirals on the dessert plate. So I sketched random spirals and painted them in sepia watercolor. Then I scanned it into Photoshop.
I rotated the file several times, dragging each new layer on top of the next until I had about three layers of spirals. I made each layer transparent by clicking "multiply" instead of "normal" in the mode drop down menu.
But how do I get the black spirals to be white? I inverted the black line-work to white line-work over another color (blue in this case, but it really doesn't matter because the blue color disappears):
I held "command + I" (ctrl + I) to invert, and the black line-work turned white (with a black background)! I renamed that layer "spirals".
At the layers panel I clicked to make a new adjustment layer, and for that layer I selected blue as my solid color background, painted that layer blue, and named it "blue". Then I dragged the blue layer to the bottom, under my "spirals" layer (It again appears black with white spirals).
Then I went up and set the mode to "screen," and voila! White spirals over blue!
Now This "spirals" layer is ready to click and drag over to any file (in this case, my chocolate mousse picture). And it will be white spirals with a transparent background so that my chocolate mousse will show through! Of course, I used some tools for additional steps to adjust, erase, burn, & dodge the layer to get it the way I liked it.
I hope you enjoyed this. And now, here is a fun little math lesson about spirals. Go to http://www.youtube.com/watch?v=ahXIMUkSXX0 for the direct link.