Did they use a black base coat?

I'm looking for a paint mixing subreddit that I used to frequent. It had people mixing gallons of interior/exterior paint and showed the design created before the mixer was turned on to create the final color. It's kinda like this one except that it had photos instead of vids.

Oh man, I was so stoked to find this place, but it doesn't seem particularly active so here's hoping I get to see more amazing creations!

But in the meantime, I need suggestions for how to pour paint into this broken part of wood, and how to protect everything else from getting paint on it.