The whiter necklace is akoya pearls.
I think the yellow strand is a nice imitation strand. I have imitation strands with a clasp marked Japan also. The pearls are about the same size as the white strand, but do you see how much larger the knots are in the yellow strand? This is not unusual with imtation strands since they tend to have larger drill holes. (Real cultured pearls are sold by weight so they tend to have small drill holes, to preserve weight.)
If they were not imitations I would guess dyed akoyas, but I do think they are imitations.
Do this simple test: rub two of the pearls together gently (I suggest using 2 end pearls). What do you feel?
Imitation pearls glide smoothly, while real nacre feels gritty. Compare what you feel when you repeat this test with the white strand.
If still unsure, and if you happen to have a 10x jeweler's loupe (magnifier) have a look at the surface of the white strand vs. the yellow strand.
Real nacre looks quite smooth. Imitation pearl coatings have a somewhat coarser appearance.
Real pearls look smooth (at 10x magnification), but feel gritty.
Imitation pearls look a bit coarse (under 10x magnification), but feel smooth.
