This should do the trick:
A = rgb2gray(imread('image.png'));
A = cat(3, A, A, A);
s=size(A);
for i=1:s(1)
for j=1:s(2)
if(A(i,j,1)<=230 && A(i,j,1)>=210)
A(i,j,1:3) = [255 0 0];
end
end
end
It will convert any grayscale sections in the range 210 to 230 to red. You could also add another similar if statement if you wanted to also add an additional condition to make some of it blue.
