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I'm pretty sure they added paint to tint it.

Was this the one you were thinking of? https://www.hometalk.com/40670621/everything-you-need-to-stencil-your-tile-floors-for-less

You can ask your question over there or look at the resources list.

Everything You Need to Stencil Your Tile Floors For Less

General FInishes makes a black water based poly acrylic.

https://generalfinishes.com/wood-finishes-professional/water-based-paints-glazes-and-pigmented-top-coats/enduro-pigmented-black

You can just use acrylic paint and add the clear coat at end. You'll want to coat the entire floor at the end anyway.

They mixed paint in with the polycryclic.

paint and then poly on top

You'll need to locate the original post to get the most accurate answer but they probably used black acrylic paint. Adding paint to a polycrylic creates a type of thin glaze.

Hi! You can also buy it. General Finishes makes a black topcoat. Stay safe!

The most difficult part is locating a water base or compatible stain to add to the polycrylic but you can also use paint and paint tints.

Here is some info for you

https://www.youtube.com/watch?v=kKeSQQ3zz4w

It is available at Lowes and HOme Depot according to what I found you can order it from your local store online and pick it up or they will bring to your car

Hi Michael, hope this post helps you out, I am still trying to locate the post you saw

https://www.youtube.com/watch?v=FkEz72VHbGo