Neon Sign: The Letters Are White
The letters are white. Every bit of color is in the shadow behind them, which is why this reads as glass and gas rather than as colored text. Four shadows do it, at 4, 12, 28 and 60 pixels: the first is white and tight so the glyph keeps a hot core, and the other three are the hue, each wider than the last. Stacking them is what makes the falloff, since a single large blur only ever gives a flat halo. The border is the same tube bent into a rectangle, the shadow stack repeated as box-shadow outward and inward so it lights both the wall behind it and the air it encloses. One custom property carries the color, so a swatch sets --neon and nothing else.