My 5888 appears to have been brown. I don't have any other examples of the dark red knight saddle except from 5731. I do think it must have a unique number from the regular red one. PlaymoDB just has the most likely number in this case and not a certain one. PlaymoDB does list one knight saddle without a picture or color, but it is actually the silver one. So a mystery until they reuse it.
Great point. Assuming that this saddle is exclusive for 5731, It will be a nightmare for me to find a spare piece for my last 5731 knight (which has the wrong saddle - I have bought it used) - as hard as finding the ultra rare gold saddle from 3837 set.

Lets hope for Playmobil to use it again!! Even better if this new set contains instruction sheet so we can confirm its part number.
By the way, exactly same thing hapened with 5731 belt loop (also in the same dark red color). At the begining, without any deeper info, PlaymoDB thought we were dealing with 30201940 belt loop (light red), but then, after new sets reusing it, we could confirm it was in fact a different belt loop - 30245160.