Locktite 243 "Blue" threadlocker is medium strength. Medium locktite can be removed without using heat. This is a great all-around RC threadlocker.
Locktite 262 "Red" threadlocker. High strength. Requires heat and a hand tool for removal.
Locktite 545 "Purple" is a high quality thread sealant also known as pipe dope.
Locktite 609 "Green" threadlocker is a retaining compound. This is intended as a permanent fix. Apply this to screws you do not want to be removed... ever. 609 is a good general retaining compound filling gaps up to .006".
Locktite 620 "Green" retaining compound for loose fitting threads. Fills gaps up to .015". Permanent.