I tried it out, and also couldn't distinguish the colors.

> Just to clarify, do you mean for the semi-transparent pieces (the generated ones without "locking in") or even the manually placed pieces?

Are these visually distinguished somehow? Are there four different colors of pieces?

The piece color also seems to be different depending on the background square color, and I wasn't able to distinguish what is what.

Yes, they're distinguished: the pieces you place there manually ("fixed pieces" are opaque and thus should have the same color, off-white or dark-grey, no matter the background square.

The pieces the AI adds when you press generate, are translucent (and as such will be affected by the background color). Then you can press "Lock in" to convert the generated pieces to fixed pieces, in which case they will become opaque.

I realize that for anyone less used to the color scheme than me, this distinction is problematic since there will be no less than six different piece colors (white, black, translucent-white-on-light, translucent-white-on-dark, translucent-black-on-light, translucent-black-on-dark)