I’m trying to write a song using the key of C with capo on 2nd fret (my understanding is that this changes the key to Bb) my question is which scale do I use to solo over this with/without capo? As an experiment if I play on garageband (with capo) and try simple chords on say the piano it sounds terrible even if I change the key to Bb. What am I missing, some logic method is escaping me 😄
Trying to understand the question! If the guitar is tuned standard, and you play an open C chord, it'll produce a concert C chord. Put a capo on 2nd fret, and play that same shape as if the capo is the nut, it'll produce a D chord.
If that's the case, and the chord is the same as the song's key, you're now playing in D. Thus any solo on any instrument needs to be in concert D also.
With a remote chance that you capo 2nd fret, and play a 'Bb chord', then you're actually playing a C chord. Can't think of any reason why you'd do this, but you're back to square one - concert C, thus any solo will be in C.
As said in comments, a capo will take the pitch/key up by the same number of semitones as number of frets. Thus 2nd fret, plus two semitones (or one tone, e.g. C>D, F>G, Bb>C).
Most use of capo is to keep open shape chords, or change key easily and quickly.
Even without capo you can play on guitar in any key. Capo is just to make some chords easier to grab and to change the key of whole piece easily.
Determine in which key you are playing without capo and then add X number of semitones where X is the fret on which you put capo. This way you will get the new key :-).