I would convert from RGB to HSV, then take the S channel. I'd then take the mean along rows (direction 2), then take the location of the hald way point. Something like
hsvImage = rgb2hsv(rgbImage);
verticalProfile = mean(hsvImage(:, :, 2), 2);
threshold = 0.5 * (min(verticalProfile) + max(verticalProfile))
topLine = find(verticalProfile > threshold, 1, 'first')
line([1, size(rgbImage, 2)], [topLine, topLine], 'Color', 'r')
If it doesn't work, attach the image that it doesn't work for.