The following guitar finishes can be unlocked to purchase from the shop in career mode:

Unlockable - How to Unlock

Blue (Synapse) - Earn 5 stars on every song on expert mode.
Custom Cherry (ES-335) - Earn 5 stars on every song on medium mode.
Custom Rising Sun (X-Plorer) - Earn 5 stars on a song on easy mode.
Ebony (Firebird) - Earn 5 stars on a song on medium mode.
Orange (Vanguard) - Complete every song on expert mode.
Pale Yellow (Moderne) - Earn 5 stars on every song on easy mode.
Rootbeer (Les Paul Double Cutaway) - Earn 5 stars on every song on hard mode.