Dye. I like Chestnut Spirit Stain (form Craft Supplies) - it's alcohol based with a bit of shellac. But have also used other water-based dyes.
Here's a small (I think it was about 2") cottonwood hollow form I did a while back
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It's just layers of red and yellow dyes. Some people will apply dye then sand it back to expose some of the wood (the dye soaks in some spots more than others). I usually just do layers and blends.
And here's an ash apple - less figure but you can see the grain still thru the dye:
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