Does the filament usage calculation on the print screen in Bambu Labs account for purging and color changes too - or is it only the filament usage for the model?
It should be calculated by running a simulation of the print. That is how the preview is generated and all the different information screens about speed, flow, etc.
You will even see the start code such as printing the nozzle fill line and such.
In preview while looking at the line type panel, turn off all but travel. You will see all the travel moves during start.
I don’t have an AMS, so I don’t know how the color changes and purging show up. They should be additional fields in the line type preview.