You can use logical indexing with the matrix of RGB values to determine which pixels are red. A "pure" red will have RGB value (255,0,0), but a red color may be considered within a range around that. To help illustrate, refer to the example code below. Note that you may need to tweak the precise pixel intensity thresholds for each channel, as per your application-specific requirements and approximated range for what is considered red and what is not.
%%Load image
rgb = imread('peppers.png');
figure(1)
imshow(rgb)
%%Find red points
redPoints = rgb(:,:,1)>=130 & rgb(:,:,2)<=60 & rgb(:,:,3)<=100;
percentRed = 100*(sum(sum(redPoints))/(size(rgb,1)*size(rgb,2)));
fprintf('Image has %d red pixels\n',sum(sum(redPoints)))
fprintf('Image is %.2f percent red\n',percentRed)
%%Highlight red on image
rgbRed = uint8(cat(3,redPoints,redPoints,redPoints)).*rgb;
figure(2)
imshow(rgbRed)
