Comparison of 10 different filaments, all the swatches are 2mm thick, 100% infill, slightly overextruded. They were all printed at 30mm/sec, 0.15mm layer height, 100% infill, and 240°C, except for the Zen Toolworks Flexible which was printed at 215°C.
Getting the right extrusion rate makes a big difference with transparency, and I did not optimize the extrusion for each of the filaments (I set them all to overextrude a bit). If I repeated the test, the results would probably be significantly different. Click on the picture above for a more detailed view, you can see some interesting effects of extrusion repeatability on transparency.
The color casts in the samples are more subtle in person, at least under artificial lights.
Thanks to PrintedSolid for the sample of Colorfabb nGen.