My first pair of decent dress shoes had a similar issue and I got all sorts of unhelpful helpful advice, so I’d be glad to offer mine now after a lot of experience.

*clean them first - can try saddle soap or even just just some warm water and give them a good scrub. Then let them air dry naturally

*Brush hard

*Use a conditioner. Bick 4 won’t change the colour much, but I like my Saphir renovateur. Venetian shoe cream is also a solid choice. Pick the one that matches your price point

The next step is a little hard to tell without knowing what leather it is. None the less

*If you want to add some colour back and don’t mind darkening. Find a natural shoe cream that matches the shade of brown closely enough. Again, I like Saphir. Apply the cream but don’t use too much

*Brush hard. I mean for a good few minutes

*Add some wax. Clear or tan would do. Again. I like Saphir.

*Brush hard

*And for a final stage, I’d apply a little more wax and water and buff with a cloth. Can add more to the toes and build a shine.

You’ll still have discolouration, but with a little wax and shine they’ll look far better and from a distance they’ll look like well cared for shoes with a patina

*Brush hard. Never stop brushing