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In practice mode there is a visualisation about upcoming chords and how they should be played. But for some single-finger chords it is suggested (by color encoding with orange) that they should be played with index finger.
But I've just found out that chords C and Am are not supposed to be played by index finger, which makes sense.
C: ... 8.png?dl=0
Am: ... 2.png?dl=0
If it indeed is a bug, could it be fixed please?
I don't know now that I can trust even two- or three- finger chords.
Fingerings in Yousician are just suggestions so you can use what works better for you.

Btw, I often play C or Am with my index finger - not sure who told you that this is not appropriate and why. But It is sometimes more comfy to play with other fingers when transition to another chord will be more comfy and smooth.
As was said previously, the fingerings are only suggestions. I have probably used all four of my fingers to hold a C and at least each of the first two to hold an Am. It depends on which chords precede and follow it. If a C follows a G then I roll my ring finger onto the bottom string and lift the other fingers. If I am going from Am to C then I use whatever finger is next to the one that holds the Am.

In the end, you just use what feels best within that song, the strings don't know which finger is holding them. But I know where you are coming from. I often don't follow the suggestions even on the early songs.
Changes to songs??

Took a bit of a break and came back to start playi[…]