I often use the same thread for piecing and quilting and only adjust the color. For piecing, I try to stay with neutrals but sometimes I will use up leftover colors in the bobbin.
Sometimes I use a thicker thread in the top and a thinner one in the bobbin. The color depends on the project.
I also like monofilament in the top with a 50 weight cotton in the bobbin.
Honestly, there are some spools I don't know what they are. I still use them.