For Mary Poppins, Disney used the sodium vapour process to get monochromatic light into a narrow channel for matte from a light splitting prism.

https://en.wikipedia.org/wiki/Sodium_vapor_process

It's charming. I'm sure digital post offers many advantages. Van Dyke might be one of a few who has done both.

He sold his Amiga not too many years ago. It may have been on eBay.