40 Transparent / Trans-Clear
This color has always been around, from the early 1950 Mursten to Town Plan, Samsonite, modern sets, and everything in between. The shade has changed somewhat over time; older ones tend to be a bluish-green or yellowed, depending on the plastic and the era. For several years, clear parts, especially noticeable in Pick-a-Brick, have been inconsistently off-white (yellowish tint), but Lego seems to have fixed that for the most part.
Does not glow under a UV light.