To fix it, change the nipple layer property to 'Difference' so that you can see in which way the nipple is different from the underlying layer. I use Paintshop Pro, not sure of the equivalent in other programs ?
You can see blue areas and brown areas now in the picture below. The trick now is to make the nipple all one colour, either blue or brown doesn't matter as long as its all the same colour.
To do this use a lighten/darken brush or dodge/burn, whichever you prefer using, to change the blue/brown on the nipple.