I refinished an organ project around 2000 using Minwax® Red Mahogany oil stain (top photo). It kept that nice red finish color for at least six years, then the red turned brown. From 2006 to today, the huge change is obvious (bottom photo). This instrument is kept in the house under normal lights only (no sun). The entire piece looks the same, color-wise; it’s not confined to just one side or area of it. – Greywolf2018
Tim Inman: Red dyes are classically light fugitive. It is nearly impossible to get a red that will last. Also, as wood ages, it has a tendency to turn brown. So, one thing coupled with the other yields the results you are seeing. Short of some Draconian efforts to get that red you like — efforts that will ultimately fail, given enough time — I suggest you just learn to enjoy the color you have. This brings to mind a memory: I once refinished a desk for a University of Wisconsin professor. It was a nice job, and I really appreciated the work. As a special “thank you,” I added a nice leather desk set for him when I delivered the piece. There were no windows in his office. The light was just ordinary fluorescent tube lighting. After only a few months, I got a call. When the staff cleaned his desk (this would lead us into another long story about how absent-minded professors need help!) they lifted the desk pad to dust the desk top. Guess what? Under the pad it was still the same red mahogany you show in your first picture. The rest of the desk was browner. They feared the desk pad was bleeding into the wood and causing it to turn red. Not so.