The chords used during the solo should be in the same key as the solo. So if your playing in C throughout the song, and youve written a solo in D, then step the chords up to D. So a C-Am-g progression would become D-Bm-A. An example of this is in "And I Love Her" by the Beatles. The song is played in the key of E but for the solo it goes up to F and stays there for the rest of the song. Hope that helped.