All of the above. Sometimes I use same top & bottom. ( sometimes it helps make things sew better)
sometimes I may use two different threads same color....maybe I have connecting threads cotton wound on a bobbin then find I have the same color in aurifil. ( just an example)
sometimes the back or bottom ( bobbin) threads is going to show, I need a different color than on the top. Sometimes I'm just using up bobbins and the colors don't matter, sometimes I'm using up spools and again ( when piecing) the color doesn't matter. It just depends.