How to address a ROSCO DMX Iris to colour source console?

One Channel (Ch) is one Light.
It doesn't matter whether that light is just a dimmer, or is the latest all-singing-all-dancing moving light.

DMX Irises (and gobo rotators etc) are "Build your own moving light", piece-by-piece.

You should patch the Dimmer and the Iris to the same Channel (fader).
The channel fader then controls the Intensity, and the parameters adjust the Iris (diameter) of the beam.

You can then record looks with both Intensity and Iris to Cues and Playbacks as needed, in the same way that you'd control a moving light that has an Iris.