Mostly #1, but occasionally #3.

I just use the same thread on the top as in the bobbin, usually a white or off white for everything, even quilting. I don't really know why, so I'll say probably "just because!"

I coordinate my thread colors with the fabrics I'm using. My usual weight thread is either 40 or 50 weight. Sometimes I get really creative and work with the variegated threads on my quilt top while quilting. The bottom thread usually matches the bottom fabric.

I'll use a different color bobbin thread when piecing if I have a partial bobbin to use up.

When quilting, I always use the same thread top and bottom because pokies show if I use a different color.

It depends on the project for sure:)

I use the same type/weight of thread for the bobbin as on top. The reason for that is just that it has worked for me and then I don't have to buy as many different types of thread.

Whether I use the same color depends on the situation. For things where the thread won't show, I often use up old bobbins and they usually don't match. For quilting, I'm usually trying to blend into the fabric, which frequently means using different colors on top and bottom.

I do all three ways depending what is wound on my bobbin. Never have any problems using different threads.

I use different threads for piecing. But I use a thin thread in the bobbin no matter what type I use on the top.

Usually #1.