If you are using hotshoe strobe, you are probably familiar with the Rosco Swatchbook. At roughly 7 US Dollars this wonderful swatch book let you gel your strobe to any possible color. If you are lucky, you can get one from your local cine or photography store, but even if not, it is still a good purchase. (Way, way back, giving it away for free almost collapsed Rosco samples plan when thousands of photographers asked for a free sample. They solved this huge demand with the Strobist Gel Pack, which got an update about a year ago).
If you were wondering how they make those sweet sweet little books of color magic, Rosco just released a short video showing how they collect the gels and convert them into a pack.
Surprisingly it is a labor intensive process (which explains why they stopped giving them away for free). It starts with collecting all the individual gels from something that looks like gel heaven and stacking them together. Then they go through a guillotine to cut them to size and have a small spigot placed at the side to bind them together.
P.S. – I know what you’re all thinking, why, oh why they are making this darn spigot hole on the side….