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So, you should type out the Scalar value. I have no idea how it interprets a hex value, but it's almost certainly not what you want.
circle(md2, p, 50, Scalar(255,255,0), 2, 8, 0);
This makes it easier to see why. Your images only have one channel. So only the value in the first parameter of Scalar shows up. So if it's a zero, then the circle is black.