There is only one reason you can't get by with one HVLP for colour and clear, and that is laziness.
If you can't clean a gun out properly from the colour to clear phase, then you're not cleaning methodically.
It is a matter of wash, spray, wipe and check over optically. Repeat if required.
I've done many a spray finish on Automotive full resto jobs on the one HVLP and cleaned the gun thoroughly between the colour and clear coat cycle. It's not hard , it is nothing more than common sense.
And what's more .......if you're spraying a job in, I dunno, let's say red mica, then you spray your clear coat over that with the same poorly cleaned gun, all you'll get at worst is a small dose of tinted clear.
The problem only occurs to detriment when the colour of your previous job , let's say blue, comes out in your next job, which just might be a white pearl job.
And that can/will and does occur in any gun earmarked only for colours
cheers,
Stu