How can I convert an RGB image to grayscale but keep one color?

Question

I am trying to create an effect similar to Sin City or other movies where they remove all colors except one from an image.

I have an RGB image which I want

Answer 1

figure
pic = imread('EcyOd.jpg');

for mm = 1:size(pic,1)
    for nn = 1:size(pic,2)
        if pic(mm,nn,1) < 80 || pic(mm,nn,2) > 80 || pic(mm,nn,3) > 100
            gsc = 0.3*pic(mm,nn,1) + 0.59*pic(mm,nn,2) + 0.11*pic(mm,nn,3);
            pic(mm,nn,:) = [gsc gsc gsc];
        end
    end
end
imshow(pic)