These can be hard to tell apart because I found there is only one flash angle where they differ.
They look the same to me at most angles. Here is 506 on index and ring, 505 on middle and pinky (over black, of course!):
(Bottle is 505)
Here are the two in the bottle… 506 on top, 505 on the bottom:
But this is the reason why 505 is my favorite! I prefer the greenish blue flash in the 505 over the midnight blue flash in the 506.
I’m holding the 505 in this picture so you can really see that gorgeous teal-ish flash toward the top of the bottle and on my middle nail:
So it just depends on what shade of blue you prefer if you can’t decide between the two of these. 🙂