How to properly use Cycles Light Groups?

Question

Recently in Blender's new 3.2 updates, we got Light Groups, however, they are rather hard to use them without knowing what to do.

How do the new Cycles Light Groups work, and how to use them?

Answer 1

Note that Light Groups are only available for Cycles at the moment.

The 3.2 Cycles Release notes describe Light Groups thusly:

a type of pass that only contains lighting from a subset of light sources. This can be used in order to e.g. modify the color and/or intensity of light sources in the compositor without re-rendering.

Here is one of many video tutorials, currently all about 3.2 alpha, but the release version isn't very different.

In summary:

• There is a new property in the View Layer tab called Light Groups. Create new light groups there.
• Lights have a new Object Property under Shading called Light Group. You assign each light that you want in a light group to the new light group here. You can bypass the first step by clicking the '+' button to create a new lightgroup, but there's a bug and the light isn't automatically assigned, so you have to assign it anyway.
• Effectively, each light group is a Render Layer pass, that contains the lighting from the specified group. You use it in the compositor to modify the lighting. The easy example is changing color.

Here's a contrived example. Suzanne is lit from

• front with a blue area light, the sole light in the front group
• back with a red area light, the sole light in the back group
• sides two green area lights, both in the side group:

Here's a render:

Here's the compositor, showing only the side lighting:

One could mix the three lighting groups in the compositor, adjusting the color balance of each separately, for instance:

That is, the color group passes can be used in the compositor the same way any other pass could be.