blue is what you should use the most. Get the gel. Removable.
red is what you should use the least. Permanent bond unless you heat it up hella
green is penetrating. Use it on permanent bolts that you don't want to take apart first (like the frame bolts, etc etc.) Comes off with less heat, but still "permanent." SUPER THIN
purple is the weakest. Use on commonly/often removed items, like timing cover bolts, or drain bolts.
BTW the colors for loctite vs permatex are the same. Permatex is a company that broke off from Loctite awhile ago, but the formulations are, surprise, EXACTLY THE SAME. The only difference is, Permatex tends to be way cheaper, and offers gels for all but the green one. The word "loctite" has become a generisized trademark, like band-aids for bandages and kleenex for tissues.