I'm sure lots of knowledgeable people have better coping tactics for colour transfer but for what it's worth, here is what worked for me.
This was an experiment I did with a white Saffiano leather wallet against indigo denim.
I took Lancome Hydra Zen day cream on a cotton bud and dotted it around the area of transfer. I rubbed it on gently with my fingers. Then I wiped it off with a Simple face wipe. I did this 2-3 times. It completely removed the transfer.
I don't know if every moisturiser would work. I've tried a few leather cleaners/balsams in the past which have not worked as well. (I find them too heavy so sometimes in soft leather like
Prada Soft Calf, I get moisture stains where more of the cleaner gets absorbed.)
I used Simple wipes deliberately because they should be less harsh than baby wipes (chemicals for wiping) and other face wipes that have brightening or exfoliating ingredients.
Colonnil is a light protective spray. I've had it recommended by Givenchy and Mulberry SAs. I bought it in the Mulberry store. It's called Colonnil Waterstop.
I hope this helps x