I use it on my light brown tack (Butet and lighter colored Beval bridle). It darkens the tack some initially, but after a few years of wear on the tack, I oil every couple of months and it hasn't seemed to darken it anymore.
I've used the darkening oil, and it doesn't darken it a whole lot.DD and I both had pumpkin colored Antares saddles that would not get darker. I ended up going to the Feibing's web site and buying leather dye. For the most recent saddle I dyed, I tried their new oil dye in "Show Brown". I just sponged it on, and it turned out beautifully. At first it looked a little 'flat' black, but it has mellowed to a very slightly reddish brown on which I have received many compliments. I also dyed the girth and bridle to match.
Using dye seems so permanent and scarey at first, but I've always been happy with the result.