Extracting RGB values from aerial photo and assigning to LAS file with FME?

Use the PointCloudOnRasterComponentSetter transformer in FME. Check out this tutorial: https://knowledge.safe.com/articles/1096/color-and-point-clouds.html

This functionality was removed from libLAS and moved over to PDAL. The filter you want to use is called filters.colorization, and it allows you to control how the bands map to the color values of the LAS file in addition to other useful switches.

See the filters.colorization documentation for how to use the filter. See the PDAL QuickStart for how to use Docker to run PDAL command line applications on Windows if that is your platform to run upon.

An example PDAL pipeline for colorization would specify filters.colorization attributes like the following:

{
    "pipeline":[
    "uncolored.las",
    {
        "type":"filters.colorization",
        "dimensions":"Red:1:1.0, Blue, Green::256.0",
        "raster":"aerial.tif"
    },
    "colorized.las"
    ]
}