Count gray and black pixels from a c++ drawing

Question

I have a drawing painted with felt-tip pens and scanned, only seven colors are present: green, blue, black, gray, violet, orange and red I need to know the percentage of color used.

I can’t tell the gray pixels from the black ones, they might help ?

I got the feed -> srcimage

Mat srcimage, hsvimage, HSV_threshold, thres_dst;
srcimage = imread("teste.jpg", IMREAD_REDUCED_COLOR_4);

cvtColor(srcimage , hsvimage, COLOR_BGR2HSV);
Mat HSV = hsvimage.clone();
int low_H = 0, low_S = 0, low_V = 0;
int high_H = 0, high_S = 0, high_V = 0;


low_H = 75, low_S = 6, low_V = 200;
high_H = 120, high_S = 48, high_V = 211;
inRange(HSV, Scalar(low_H, low_S, low_V), Scalar(high_H, high_S, high_V), HSV_threshold);

Mat binfinal, binfinalsrc;
bitwise_and(HSV, HSV, binfinal, HSV_threshold);
bitwise_and(srcimagetemp, srcimagetemp, binfinalsrc, HSV_threshold);