Mixing actual mercury vermilion into lacquer:
(notice how it results in a paste that must be kneaded, then liquid lacquer is added in two more steps until a sort of thick syrup is achieved)

https://www.youtube.com/watch?v=09fWbdlCrUs

Here the urushi is mixed with a rice flour pudding:

(And I love how he uses his little bamboo scraper to clean his bowls and spatula and not waste a drop)

https://www.youtube.com/watch?v=_IE7ir4hvmY

The first few minutes are of interest the way it shows how the lacquer sap is mixed with colorant and - something liquid? What?

https://www.youtube.com/watch?v=Ao_geAaxjgs

There seem to be about 50-60 videos in this series. I recommend using the scroll button liberally, lest you find yourself gazing at a screen for ten minutes watching someone stir something. Skip to the action parts. [Unless, of course, you're baked and watching paint dry is just a fine way to chill as you work your way through a bag of cheetos]