IMO, aqua fortis or lye water will both not only NOT make walnuts grain "pop", but it will bury it instead.
Most good walnut doesn't need any agent that will darken the wood. It will be dark enough all by itself.
Some people do not understand that raw, dry walnut looks very light. Almost like a finished cherry.
Dry walnut is very deceiving.
If a builder wants to see what the walnut stock will really look like, all they have to do is to apply some water to the surface.
The wet wood will instantly change from its dry light color to a rich, dark, beautiful wood, often with darker stripes appearing in interesting patterns.
This is why I always tell people, "Don't even think about using a stain on walnut until you have wetted the stock to see what your really dealing with."
Occasionally, some sapwood will be present.
Sapwood is notably lighter even when it's wet.
If sapwood is showing then, yes, get some alcohol based Walnut stain. I like Birchwood Casey's Walnut. Dilute this stain about 50 % with alcohol. Then wet the area so the heartwood changes to its dark color and apply the stain to the sapwood until it matches. This may take several coats of diluted stain but with a alcohol based stain you can apply as many coats as needed to darken the wood.
As for maple "popping", as others have said, that's already built into the wood but the stain does greatly help the stripes show.
Those stripes are areas of wavy grain.
Where the wave is cut/sanded off it will absorb a lot of stain. The area between waves don't absorb as much stain/aqua fortis so they stay fairly light.