1) Yes, Colorless is a Pokemon Type and affects the damage done with Rainbow Force.
2) Every unique type present on the Pokemon on the bench is counted, so yes, Rainbow Force could increase damage for dual type Pokemon.
For example if you have Golurk from Ancient Origins with Double Type ability, it would count for 2 types in a single Pokemon and contribute 60 damage. If you have Keckleon from Rising Rivals with Colorful Body Poke-BODY, it would count for 9 types in a single Pokemon and contribute 270 damage.
Each different type is counted. Duplicate types, for example Golurk (Dual Psychic and Fighting) wouldn't increase damage if there were other Psychic and Fighting types on the bench.