Cleaning the hide properly is vital particularly before using any form of leather dye: ~ purchasing the correct cleaner and not saturating the leather is also as important.
The process is tedious but you will never get descent results unless done properly. From what you have described I would think that colouring the leather with the appropriate dye matched to existing will give you the uniform finish once the cleaning process is accomplished.
The last recommended company will supply all your needs and rather than typing again I would take a look at various sites to include -
https://www.flyingspares.com/uploads...STRUCTIONS.pdf
Trust it helps.