Digital Light & Color

How do i do this ... for instance, working on the eyes of a cat, intensifying the color ... or even changing the color?

There is a tutorial that you may find useful. On my machine it is here:
Program files/Digital Light & Color/Picture Window Pro 6.0/02Tour.pdf
This tutorial has been around for a long time, way before version 6.0.
I can't really remember, but I suspect that the tutorial is automatically put in that folder as PWP is installed.
Mike.

The basic idea is to create a mask to isolate the eyes using the Mask Tool and then, with the mask overlay still on the image, use one of the transformations such as Color Saturation, Color Balance or Color Correction to alter the color. You can check out either the electronic manual or the help file for more details.