I am reading Brian Solomon's Classic Railroad Signals (c. 2015) and thoroughly enjoying it. I was surprised to learn that into the early decades of the 20th Century, a number of railroads employed a white light for a clear indication, with green used as a cautionary aspect! This was the R-G-W (Red-Green-White) standard and was a legacy of an earlier era using lanterns and flags. Despite an industry-wide adoption of the current R-Y-G (Red-Yellow-Green) standard in the early years of the 20th Century, several roads carried the antique R-G-W standard into new state-of-the-art signal installations in the teens, including both the WM in 1915 and the MILW in 1915 on its Pacific Coast Extension. The WM soon revised their choice, changing lens colors and rule books to join with the new R-Y-G standard. I'm uncertain about the longevity of the MILW's choice, but by the mid-1920s, the MILW led the industry with nearly 600 Miles of line protected by colored light signals.
The White, or "Lunar" aspect is still in use on some railroads today, isn't it? I seem to recall someone posted a pic of a few on here before, maybe?
You're right, but Lunar has a lovely whiteish, pale blue aspect and is a restricting indication, often very slow. In the case of the old R-G-W standard, the aspect for clear was a pure white light. Yep, lunar signals are indeed in use. I recall seeing a lit lunar signal on the former MILW main south of Milwaukee perhaps five years ago. I caught this shot last year of one on the former SAL. Amtrak's Silver Star was stopped ahead and the lunar aspect was displayed to allow a CSX freight to advance on the Star's markers, which the freight did, but very slowly at no more than 10 MPH. My photo didn't reproduce the lunar color well.
