With frameA() you need to pass in the row and column, in that order, not x and y in that order. So you'd need frameA(y, x) but even then you'd need to convert y and x to integers because you can't have fractional values: frameA(round(y), round(x)).
To set the values in the rectangle to a particular gray scale, you do something like this
[rows, columns, numberOfColorChannels] = size(frameA);
if numberOfColorChannels > 1
% Convert to grayscale if it's color.
frameA = rgb2gray(frameA);
end
col1 = round(Position(1));
col2 = round(col1 + Position(3));
row1 = round(Position(2));
row2 = round(row1 + Position(4));
frameA(row1:row2, col1:col2) = desiredGrayLevel;
