Changing parts of Bitmap

[Sahil Reddy]

I have 2 same images but one is the coloured version of the other.

Now what i want to achieve is the following

User draws Circle , i want to show the image inside the circle as coloured and one outside the Circle as black and white only.

i did canvas.draw for coloured and and black and white so the black and white is over the coloured , now i want to make above black and white thing transparent for inside circle and make the lower coloured
picture to be visible . I am unable to do the same.

Please assist

Regards