We made a video about the basics of ND gels the other day, and given that we don't have a grip truck to haul stuff off of we had to go actually buy gels from the photo supply store. I took the opportunity while there to grab another of my favourite things on the earth, a little sample-book put out by Lee Filters (Rosco makes them, too) containing little swatches of all the gels and scrims and reflectors that they make, which is a lot of different gels, scrims and reflectors.
Each sample is about 1" x 4" and it comes with a little card that's got its name and its spectral transmittance, which if I was smart I'm sure I could use to draw some conclusion I couldn't otherwise reach by actually looking at the gel. I love flipping through this book thing because it's full of stuff I've used, practically speaking: NDs, diffusion of all sorts, CTO, CTB, reflectors and such, but it's also filled with hundreds of samples of weird stuff that lets me pretend to be Dante Spinotti: "I didn't ask for C.T.B., I asked for Lee #723, Virgin Blue, and if they don't have that, I want Shanklin Frost or Old Steel Blue."
Man if I had a bunch of dough I would make a seriously colourful movie is what I think I'm getting at