Color Specific Points in a Meshgrid Matrix

I am trying to color specific points/coordinates created in a meshgrid matrix, created below:

[LAT LON] = meshgrid(lat,lon);
 a=LAT';
 b=LON';

plot(a, b, 'o','MarkerEdgeColor','k','MarkerFaceColor','k');
axis([-18 18 -18 18]);
hold on

What I have so far plots all points black. The meshgrid creates a 51X51 matrix. How do I go about coloring all points in the plot red if b= 10th row, for example?

Thanks.