I have a dishwasher safe set of boards. I've heard a common advice to use separate boards for meat, fish and ready to eat food. However it got me thinking - I don't mind mixing plates or other utensils as they are cleaned by dishwasher anyway (or hand washing).  Does the advice applies to mixing without cleaning? Or are there reason not to mix despite cleaning? 
I always considered the advice to be like a second level of defense: Proper cleaning should be enough to make the board suitable for any task. And not everyone has multiple boards, just saying.  However, nothing and nobody is infallible. Using separate boards for various uses helps in the rare cases when something went wrong or someone did a sloppy job. In shared kitchens, color-coding also tells the other users, which boards should be washed especially carefully and which one you shouldn’t absentmindedly use for chopping up a salad just because somehow someone left it out on the counter. The price of an extra board is usually low enough that it’s worth the investment.   So in short:   Mandatory: No (except for commercial kitchens) A good idea nevertheless: Yes.  