One other little tidbit about USGS maps and their idiosyncrasies. Natural features like land contours are printed in brown and water features (springs, creek, etc0 are printed in light blue. Cultural features (houses, roads, etc are often depicted in black. In using the map you will often find that the map was produced some twenty or thirty years ago. The result is that the natural features, brown and blue, are often very correct, while the cultural stuff is outdated - roads are built, cabins are torn down, etc. Maps get photo revised (the revision are depicted in purple), but even that doesn't happen consistently. The upshot is that you can rely on hills and crags being there, while the road or trail on which you are traveling may not be shown at all.

In my corner of the world, I used maps older that most of the members of this forum (made in 1943); the landscape was fine; roads, etc. were laughably inaccurate..

All this stuff about different datums is a bit arcane and technical. Its relevance depends a great deal upon the purpose for which you are using the map. If you are traveling and trying to reach the proper summit, pass, or drainage, it probably doesn't matter. If you are trying to located or find an archaeological site or fossil locality, it can be critical.

The thing about the different colors (red works its way into the scheme for showing dense urban areas) is that the USGS has good taste and the maps make great wall decorations - at least in my opinion. Others, like my wife, may not agree......