User should be able to pick a Gradient color and apply them linear or circular from the touch point taken as the origin.

I want to make a coloring game in Unity 3d. There is a grayscale 3d environment in the game view. When the game started, user can pick a color from color picker. After the user clicked (touched) an object in the 3d environment, that object is get painted from the picked color. Here, what I am doing is, when object is clicked, I change its background color to the picked color from color picker. But I want the picked color to apply to the clicked (touched) game object linear or circular from the clicked point (touched point) taken as the origin. I appreciate if someone can answer to this.