Question

How can I specify something like: if the plotted value is < 10, color it red, if it is >20 color it blue, if it is in between color it black?

Answer 1

Simple example:

x=1:0.1:30;
y=x;

hold on
plot(x(y<10), y(y<10),'o','color','red');
plot(x(y>20),y(y>20),'o','color','blue');

the y(y<10) is logical indexing, and is the key to this.