How do you know when to change a gillette mach 3 turbo razor? please help!!?

so i thought when the blue strip on the top of the razor blade turned white you had to change it? i just shaved with it and the blades were really dull and i cut myself a bunch of times and it was no where near as good a shave as i usually get so i changed the blade. i noticed the new blades strip at the top is already halfway white! does the strip tell you when its time to change the blade? how do you tell you need to change the blade before you try it and cut yourself and get a horrible shave?