From Friedman's chapter on 'Gasparcolor and Silver-Dye Bleach':
The preparation of a monopack suitable for two-color work, has also not been overlooked (USP 2028279). But instead of coloring each layer with a single dye, two are used. Thus the orange-red layer will contain pyramine orange and azo fuchsin, or azo fuchsin and mordant yellow, while the green layer will contain benzo pure blue and pyramine orange or benzo pure blue and metanil yellow. The rate at which the two dye components are acted on by the bleach solutions varies with the dyes. Thus it becomes possible to obtain dichroic effects in each layer, orangy highlights with deep red shadows in the orange-red layer, and sky-blue highlights with deep green shadows in the other. It becomes possible in this manner to obtain pleasant flesh and sky renditions, and at the same time, maintain true blacks in the middle tones & shadows. Such a process could be termed "two and one-half color."