Yellow lenses have been used in safety glasses/goggles on construction sites for ages.
A few of the construction sites i have worked on have supplied three pairs of safety glasses, a clear lens for everyday use, a tinted lens for bright sunny days and a yellow/amber lens for dawn/dusk and low light working.
As gassy mentioned the yellow lens is great for woodsball when there is less light as they sharpen your view.
And for those that want to know the nerdy science reason for this, its because the yellow lens blocks out blue, grey and ultravoilet light.
This means your eyes will no longer see the hazy sunlight shining through the trees that usually makes it difficult to see when playing in the woods and you will have better vision in foggy weather as your eyes cant see the light reflecting off the fog.
And they work the opposite way as tinted lenses do, instead of making everything look darker, the bright yellow lens makes things look brighter than they are, ideal for low light situations.