For quite some years I purchased high quality, concentrated leather stains in a fur- and leather shop in Copenhagen.
They are in glass vials, seem to last forever and do a perfect job on my briars. They are basic colours and I mix them if need be.
Fiebing's is fine too, but I believe less concentreated. Never mind, the stuff goes a long, long way.
I hope I'm not colourblind and stain pens never worked for me. Messy and I cannot mix the colours properly.
My OCD is a pain
There's also a little jar of "natural" alcohol stain. Every now and then I fill the bowl of a stummel in alcohol to soften accumulated stellacite hard material ( think cake ) etc. before being able to ream/clean it. Makes for a perfect deep dark brown stain.