implement eraser for drawing using NSColor and NSBezierPath

Hello,
I had an image on cocoa window. i can perform operations like
drawing with green pen ,red pen etc using NSBezierPath. now i want to
create something like eraser for erasing on window. For that i created
color from NSImage using API "colorWithPatternImage:NSImage" that
returns NSColor then later i called "set" method on that NSColor
object. but still i am not getting the exact pattern color as
background image . some newer color is getting created.


NSColor *color1 = [[NSColor colorWithPatternImage:image] retain];
[color1 set]; //set the pattern color on current graphics port
[path stroke]; //path is object of NSBezierPath



can anyone help me what is the reason for this?