I have got the impression Old English is pretty temporary, while it does have effectively a tiny bit of stain in it which stays. There are the marking pen sets with multiple shades, where you start with a light shade and progress darker until it blends. And crayon type sets with same procedure. I have replaced sections of veneer entirely missing, by applying new veneer and sanding to the appropriate shape and then using a stain that has a certain opacity, I forget the brand. It’s not just dark or light but actually has some opacity in it which can make for example a bolder grain pattern in the new veneer fit with a more subdued older pattern. Then a little lacquer. And a layer of lacquer over the whole piano to bring up the whole thing in some cases. But in general I find a whole lot can be done by just noticing which parts of a spot are too light or dark and basically painting like on a canvas. In some cases one ought not be too perfectionist as the case of a piano should be analogous to a violin case or guitar case . King Charles is known to wear coats with visible mending, and some of the best pianos are in performance spaces where they invariably have some dings. On the other hand you have people who have a perfect living room or something like that, where they’d like it perfect.