I don’t have much experience growing Paeonia obovata and P. japonica plants, they tend not to grow very well here for some reason. These are some plants grown from seeds, so perhaps not all ‘true’ (the only flowering one was a ‘pink’ P. japonica, see last picture), but there are some 5 different ones of each species and within each species they are quite consistently the same in appearance. What I do notice however (and why I’m posting this without them being mature), as they are growing next to each other at the same location, is that P. japonica tends to ‘sunburn’ easily and become somewhat yellowish pale due to the sun. Even though both are growing in shade, it seems P. japonica needs much, much more shade than its close relative (the same according to Hong, but general public opinion doesn’t agree with him)? First two images should be P. obovata alba, last three are some P. japonica plants. I think the leaflets are also rather different in shape, P. obovata being much more rounded, thus shorter and wider.