2 - Making A Digital Composite from Separation Negatives
The steps I describe below are done with Adobe Photoshop. Other image editing programs could do this, but I frankly do not know the steps for those programs. I have made a Photoshop action that automates all these steps.
After the three separation negatives are each scanned to individual files, they are assembled as separate layers in a single Photoshop document, each separation layer labeled according to its corresponding color: RED, GREEN, and BLUE. The images are inverted to a positive. They will look something like the "Inverted"-labeled images at the far left in the picture below.
The action makes an empty color document in Photoshop, and pastes each "Inverted" positive into the empty composite color (RGB) document's corresponding channel. That is, the RED Separation Invert image layer is pasted into the empty composite document's Red channel, the GREEN Separation Invert image layer is pasted into the empty composite document's Green channel, and the BLUE Separation Invert image layer is pasted into the empty composite document's Blue channel.
Photoshop internally reverses the colors as well: when RED is reversed it is cyan, GRN reversed is magenta, BLU reversed is yellow. That's why the three "Invert Pasted to Comp" positive images are displayed below with inverted colors.
Then - voila! - You have a color composite, as the example "Finished Print", below. You may not consider it finished, so you can do further corrections to the composite itself, or go back to the layered separations document, make more corrections, and redo the composite.
If you use the Photoshop actions I have provided (available to registered users), you can correct density and color before combining the three images as a new composite color document. Now I'm sure it's not intuitive to you how to correct color while working in black-and-white, so you will (soon) find further explanation for this topic on the Knowledge page.