In terms of gunsmithing, I'd have both on hand. Blue is going to be more commonly used, as most folks here have alluded to.
If you've not seen the Loctite applicator sticks, as opposed to the droppers, they are pretty frigging awesome. At least for myself, I would always cut the opening in the tube too large, and way overuse loctite-getting it not just on the threads, but my workbench, the dog, etc. And then, since the tube is so small, the loctite gnomes would come and repatriate it, leaving me having to make yet another trip to the frigging hardware store for more loctite.
The applicator sticks (think glue stick), are large enough that they are harder to misplace, solid so they don't leak, and easier to apply (at least for me).
Yes, I'm totally biased, and only wish I'd known about them sooner. They're also not horridly expensive on Amazon. Couple the ease of use, and the reduced waste (at least for me), and a stick should last a damn long time.
The ONLY gotcha with these is that you cannot retract it back into the tube once you click it...so, only expose as much as you really need for your work.
Loctite Blue Applicator Stick: http://amzn.to/1D3IZoQ
Loctite Red Applicator Stick: http://amzn.to/1MNb2kq