Color update problem

I am new with OpenGL, and trying to build a Windows application that displays, modifies, and re-displays (as the image gets modified) a grayscale image (in PGM format). Upto now, I could load/display an image, and also could modify it, eg, say, drawing a circle in it (I checked where I saved the modified image to verify that the modification actually worked). After the modification, the circle should appear in ‘white’ as I drew it in the maximum grayscale intensity. But the problem is, it is appearing in ‘black’, instead of white. Any suggestion how to resolve this?

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