Try this:
imageData = imread([yourFile]);
% split R, G, B
imgR = imageData(:, :, 1);
imgG = imageData(:, :, 2);
imgB = imageData(:, :, 3);
% get linear index of all pixels with color not white
colIdx = (imgR < 1 | imgG < 1 | imgB < 1);
% change color to black
imgR(colIdx) = 0;
imgG(colIdx) = 0;
imgB(colIdx) = 0;
% write back to new image data or overwrite the origin imageData
newImage(:, :, 1) = imgR;
newImage(:, :, 2) = imgG;
newImage(:, :, 3) = imgB;
