How to genrate gradient in floodfill?

How to genrate gradient in floodfill?

I have an PNG image with complete transparency. It has a picture of an
animal , just bordered with black color.
Now , I want to paint the image as my finger touch the Ipad Screen,but
paint should only appear inside the bordered region not out side.
I want to fill image by gradient not by simple RGB colors.
I am using Flood Fill Algorithm for fill the color.Now i want to generate
gradient in Flood Fill.How can i achieve that? I am giving RGB values in
code.How can i covert that RGB into gradient?
Thank you for help in advance...