Amazon.de actually has the black one 10€ cheaper than the white one.
Can one install Sailfish on a Nexus and then still revert to Android if something doesn't work?
I never tried installing default Android which comes with Nexus 7, but I installed Cyanogen Mod on it (which is basically stock Android without Google applications, but you can still install them there if you need them). It should be doable. I even played around with KDE Plasma Active on the older Tegra based Asus Nexus 7.
Usually installing Cyanogen Mod is done by installing Clockwork Mod recovery image, which allows flashing OS images onto existing filesystem simply sending them from your computer using adb tool (while device is attached with the USB cable). Sailfish + libhybris is installed the same way, but early Plasma Active installation was a more complicated process.
Cyanogen Mod has a good wiki which covers its installation:
https://wiki.cyanogenmod.org/w/Devices
Basically you can clear partitions which will wipe your Sailfish installation on Nexus 7 and then install Cyanogen Mod. I suppose there should be also a way to install the default Android image.
Just for the reference, Sailfish which is coming with Jolla tablet will be version 2.0, while current Nexus images are still 1.x.
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Also, Sailfish community is very helpful, and for various hardware adaptations you can use #sailfishos-porters on irc.freenode.net. I asked many questions there.