Why is Primary Symbology > Classify unavailable for some images?

[Bud]

ArcGIS Pro 2.9.5 and 3.4.0; TIFF and JPEG

With some images, Primary Symbology > Classify is available in the pick list:

But with other images, Classify isn't available:

Why is Classify available for some images, but not others?

Use case: Easily remove white pixels from georeferenced plan

[Bud · Accepted solution]

It looks like Classify only works for Grayscale images, not color images.

[Bud]

Idea: Raster Symbology — Automatically convert color raster to grayscale to support Classify primary symbo...