Here is one way:
% Some made-up data x = 1:0.25:10; y = x + 0.5*rand(size(x));
% Identify the x ranges redIndex = x < 5; blueIndex = x >= 5;
% Plot figure hold on h = plot(x(redIndex),y(redIndex), 'r.', x(blueIndex),y(blueIndex),'b.'); set(h,'MarkerSize',24) % Don't really need this, but wanted the markers to really show