NavList:

Welcome aboard!

A Davis Mk 15 is a great sextant and quite capable of getting you safely across any ocean using celestial navigation. The only serious limitation with plastic sextants is that you need to zero out (or measure) the index error before every round of sights. It's a minor nuisance, but you can expect accuracy of 2-3 miles if you're careful using it. 

There is no real logic to the shades on sextants, and on some sextants the shades are distinctly bizarre. I owned a 70s-era MAC sextant (a Tamaya-like sextant) some years ago that had one shade so dark that the Sun could not be seen through it. The various colors are intended for convenience only. You can make your Sun green if you like... or red if you prefer. But you're right: they might as well be color-less neutral density filters of various strengths. Note that the horizon shades make even less sense.

You're quite correct about slide rules. Common slide rules don't have quite the accuracy for full celestial navigation calculations, but you can use them for tasks like interpolation and some specialty sights. If you're a fan of slide rules, you should inquire here about the Bygrave and other cylindrical slide rules. These are designed to create very long scales by wrapping them around a cylinder. There has been great hobbyist interest in these here in the NavList community over the years.

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA