I was recently looking for dark brown leather straps for my black dial watches (because I got bored with black straps), but every brown strap I have bought is not dark enough. I had to figure out a way to stain them to a darker brown, most ideally something between brown and black.
I started working right away, and forgot to take pictures of the brown straps before they were stained. The original color looked like this (product image from the Amazon store that I bought the straps from):
I put about 10 drops of "dark walnut" wood stain into a tea spoon-ful of coconut oil, and stirred it until it became a dark brown paste. The idea came from some online leather-care tips, saying that coconut oil is good for leather maintenance, and can also soften it. I also happen to have some left-over wood stain.
I used a Q-tip to apply the paste onto the straps that I wanted to darken. After a few minutes, I used a clean piece of cloth to wipe off the excess oil, and cleaned the straps. The straps are now much darker, almost black, but not black. They look very nice.
If only coconut oil is used, the leather will turn a little darker for about less an hour, and will return to almost the original color after that. But with the stain, the darker color remains on the leather, which also feels a little softer now. Talk about killing two birds with one stone.
I guess with different shades of stain, leather can be dyed/stained to different colors. And it is more effective to stain new straps (fresh from the store), because we haven't rubbed much of our human oil or dirt into the leather, and the staining color will be more even and consistent. But the good thing about using coconut oil is that, if the color is not satifactory the first time, we can do it multiple times, because the oil is said to be good for the leather, right?
There are people using water based dye to stain leather, also with good results. But they use water to dilute the dye, instead of oil, and they have to let the leather sit and dry, and will need to seal it before use.