Render delegate not reacting to light color update using the Solaris light mixer

Updating the light mixer in Solaris is working correctly for the exposure, disabling and soloing lights, but not for changing the light color. Restarting the render or disabling and reenabling the light will force a color change update as a workaround but it's not responsive directly to a change in the light color.

Start with the Solaris light mixer and render delegate path; reproduce the issue by changing only a light's color while exposure, disabling, and soloing remain responsive. Trace the color-update handling and verify that changing the color updates the active render without restarting or disabling and re-enabling the light.

Written by the indexing model from the issue text.