This color of sea glass originates from clear glass. During WWI, certain chemicals became more difficult to procure. Glass manufacturers made some recipe changes to the glass, not realizing that the chemical differences would make the white glass change color over time, as it was exposed to sunlight. The sun cooked pieces will continue to darken as the glass sets in sunlight.
If you ever have the chance to take Seattle's Underground tour, you will notice that the city put clear glass bricks in the sidewalk. This allowed light to pass through to the underground while both the upper and underground sections of the city where in use. From street level, the bricks are barely noticeable and appear to be just dirty glass bricks; but from the underground the bricks are a deep lavender hue!