Mike was using alcohol based stains in the tutorial.
As you may know, alcohol based or water based stains dry after they are applied.
Because they dry, a person can put on as many coats of stain as they wish, changing from one color to another to try to achieve what they want.
There are also oil based stains available however I don't recommend them for staining firearms.
Often, when using an oil based stain the wood will become saturated with the oil and additional coats of it will do nothing.
If your asking about the black paint Mike used, that was regular oil based paint, most of which was then removed leaving very dark areas which are meant to reproduce the heavily oxidized linseed oil that is often found on old original guns. (After years, linseed oil oxidizes into a black colored finish.)
As for staining walnut, I usually don't recommend it.
Newly sanded Walnut appears to be a fairly light colored wood but when it becomes wet with water or with finishing oils it darkens considerably.
By their general nature, stains tend to cover up the natural beauty of the grain pattern the wood has.
In my opinion, this is especially true with Walnut. Just a little dark stain can make a very nicely grained Walnut look like a dark brown very plain piece of walnut.
This is why I highly recommend that before anyone working with walnut gets out the stain they really need to run a test to see what the unstained wood will look like when it is finished.
To do this, take the finish sanded wood outside into the bright sunlight along with a rag that has been dampened with water.
Use the damp rag to wet the wood and look at the color and the light/dark grain pattern.
This is exactly what the walnut will look like if you do not stain it but instead, just finish it with a good finishing oil.
If this wet test shows the wood to be very light colored, or more commonly if it has an area that is very light, then and only then should stains be thought of.
After doing the wet test, let the wood dry and then use a piece of new sandpaper to whisker the little wood hairs that will pop up.
Whiskering should always be done before staining so this doesn't add any steps to the process.
Another nice thing about water based or alcohol based stains is they can be easily thinned to make them less effective.
That allows the builder to use the diluted stain to gradually darken the wood by applying several coats if they are needed.
The person doing this must remember that the true color of the wood will be seen while the wood is wet with the stain but after it has dried it will look much lighter. It only shows its true color when it is wetted with water, alcohol or oil.
