You can't just go by color with loctite, you have to go by number... There are multiple versions of blue, red, and green, along with yellow, black, and purple, each differing in strength, viscosity, applicable thread diameter range, temperature range, oil resistance, and need for a primer or activator. And to make things more confusing there are multiple green retaining compounds for non-threaded applications like bushings and sleeves as well as multiple green thread lockers for threaded parts.
That's why going by color alone isn't enough, as a certain part number of blue loctite might not be correct for your application but another part number that's also blue can be perfect for your application.
People often just go by blue and red (typically 242 & 271) because they're the 2 most commonly available flavors at your typical automotive or hardware store, and as noted by
@msgriff above, loctite doesn't recommend the medium strength thixotropic 242 for small diameter fasteners <6mm/0.250" like you would typically find on optic mounts. 271 being lower viscosity works on small diameter fasteners but the high breakaway torque of 271 can often shear the screw or strip the head on small diameter fasteners unless you apply heat first.
For small diameter fasteners without overly long thread engagement that you want to stay put, medium to medium-high strength loctite 290 or the oil resistant 294 version are great. They can be applied before assembly as normal, but are also low viscosity and wicking and can be applied to the exposed end of the fastener after assembly if it is accessible. Both 290 and 294 hold very well on small diameter fine thread screws like on ring caps and scope bases, but they're not so hard to remove you typically need heat or risk breaking the screw or stripping the screw head when removing them unless the screw is very small or has a very long thread engagement and surface area for the thread locker to bond to.
For small diameter screws that are prone to stripping or shearing from a too strong thread locker or have a lot of surface area because of a very long thread engagement, 220 works well, much better than 222 in my experience. It's another low viscosity version that wicks and can be applied either before or after assembly.
Similar recommendations go for vibra-tite. 111 is the choice for low strength for small diameter fasteners, 150 for medium strength small diameter, and 131/132/140/141 for high strength. For the gummy adjustable stuff, the VC series works well, the difference being what solvent base is used, as for applications going into plastic you need to choose the appropriate solvent that won't attack the base material. For general use on metallic parts where you don't have to worry about base solvent plastic compatibility issues VC3 is the least expensive and the most available.
As for what to use on your rings... Up to you. If the part was designed correctly and the screw is torqued properly to begin with and not used in an environment with high vibration you shouldn't need any thread locker, but it's certainly reassuring to use it. I use a lot of loctite 290/294 on small diameter fasteners and it always holds well (occasionally too well on long thread engagement screws and needs heat to remove.) For long thread engagement small diameter stuff I use 220 with very good results. If it's something I probably won't be removing frequently or gets buried or inaccessible under other parts I'll still use 290/294/220, but lately I've been using more vibra tite VC3 on hardware that remains accessible or may get frequently swapped; the VC3 might not "lock" the screw like 220/290/294, but it offers easy disassembly and will help keep a screw from vibrating or walking out.
