Page "Color gel" Paragraph 12

It is common for a gel manufacturer to publish the transmission coefficient or even the spectral transmittance curve in the swatch book and catalogs.

A low transmittance gel will produce relatively little light on stage, but will cast a much more vivid color than a high transmission gel, because the colorfulness of a light source is directly related to narrowness of its spectral linewidth.

Conversely, the flatter its curve becomes, the closer the gel is to a neutral density filter.