If I update to Cesium 1.30 my billboards all start bleeding their colours through to each other.
Please see this sandcastle example to see what I am talking about: http://cesiumjs.org/Cesium/Apps/Sandcastle/?src=Hello%20World.html&label=Showcases&gist=73d3d23629edb117ca096324d0af7993
If you zoom in and out on that so that the coloured billboards are overlapped you will notice the green and yellow bleeding into each other for some reason. It seems like a graphical glitch.
You can see a screenshot of the issue here where there is obvious blending of colours where the globe of the right-most billboards is under the text of the left-most billboards: http://i.imgur.com/YPrRAWB.png
By comparison, this was what it looks like in 1.29 when it works perfectly: http://i.imgur.com/Sh3mtg0.png
Thanks for your time!