Prevent colour transfer on light leathers?

[whateve]

Do you have any experience of washing a NVT in Oak by Mulberry? I'd like to buy a pre-loved but they are often quite dirty... or the price is almost same than in the stores after regular discount...

I'm sorry, I don't. I've washed a few brands but have had the greatest success with vintage Coach. The leather on the Mulberry might be similar but the construction could be different. Some brands use glue, and if the bag is washed, the glue melts and stains the leather. You could ask on the Coach rehab club. https://forum.purseblog.com/threads/coach-rehab-and-rescue-club.833400/page-1428#post-33215786