Red Loc-Tite is like a wife... your stuck with her and to get rid of her, it's gonna cost you a bundle. Unlike Blue Loctite.

You put Red Loc-Tite on a bolt that is prone to losening from vibration and also would lead to catastrophic relusts if it did. Flywheel bolt is a good example but you will have to use some major effort to get the bolt out later, so beforewarned. If it is isn't so critical but you still want to make sure it won't just vibrate loose, then you want Blue Loctite.