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).