Use the color matrix and set it to the image attributes.
The color matrix for gray scale is
ColorMatrix cm = new ColorMatrix(new float[][]{ new float[]{0.3f,0.3f,0.3f,0,0},
new float[]{0.59f,0.59f,0.59f,0,0},
new float[]{0.11f,0.11f,0.11f,0,0},
new float[]{0,0,0,1,0,0},
new float[]{0,0,0,0,1,0},
new float[]{0,0,0,0,0,1}});
refer
http://www.bobpowell.net/grayscale.htm[
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