Swatch Book

When designing lights, a swatch book helps you choose the best colour for a light, as it has a sample of all the gels and frosts made by that manufacturer.

Whilst you can choose to get a Rosco International swatch books, it is a lot easier to get LEE Filters' gels, so it is more useful to have one with there samples. LEE make multiple variations of swatch books, but the main types are the Designer edition and the Numeric edition.

The Designer edition is the most useful, as it has all the samples in chromatic groupings, so similar colours are together.

The Numeric edition has all the samples in numerical order.

You can get swatch books from hire companies which sell gels, or from an online theatre retailer.

Virtual Swatch Books
There are virtual swatch books available both on the web (such as on the LEE website) and as apps for smartphones. Whilst these can give you a rough idea of colour, as well as other information, they can't give you an accurate representation of the colour of the gel.