Here's are 4 demo that all achieve the same result.
Xvals = [1,2,3,4,5,6,7,8,9];
Yvals = [2,5,5,5,2,2,5,5,5];
% Show scatter plot were all y-val greater than 3 are red
% and all y-vals less than or equal to 3 are black.
% METHOD 1
cla()
col = [0 0 0; 1 0 0]; % [black; red]
isGreater = Yvals > 3;
scatter(Xvals, Yvals, 50, col(isGreater+1,:), 'filled')
% METHOD 2 (variation of method 1)
cla()
baseColors = [0 0 0; 1 0 0]; % [black; red]
isGreater = Yvals > 3;
cmap = baseColors(isGreater+1,:);
scatter(Xvals, Yvals, 50, cmap, 'filled')
% METHOD 3
ax = cla();
isGreater = Yvals > 3;
scatter(Xvals, Yvals, 50, isGreater+1, 'filled')
ax.Colormap = [0 0 0; 1 0 0]; % [black; red]
% METHOD 4
cla()
hold on
isGreater = Yvals > 3;
scatter(Xvals(~isGreater), Yvals(~isGreater), 50, 'k', 'filled')
scatter(Xvals(isGreater), Yvals(isGreater), 50, 'r', 'filled')
