Yes, you always use the bobbin and needle at the same times. You need extra bobbins for when you change your thread color. Otherwise you'd have to unwind and rewind the bobbin each time you changed colors.
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Yes you use the same color. You just load the bobbin with the thread you're using on the top. If you were hemming grey dress pants I'd use grey thread, or black.
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You use them both because those two threads interlock, otherwise you wouldn't be sewing anything. And think of sewing with two different colors at once, you'd have them both on the hem. Not pretty. As PP said, extra bobbins are needed for different colors, or even the same, if you don't want to stop halfway a super long project and need a quick change. Although winding new thread only takes a minute.
If you are confused why two threads are needed because you only use one when sewing by hand, keep in mind that the needle "by hand" can go all the way through fabric and come back up the other way, but that can't happen on a sewing machine.
You can use different colour needle and bobbin threads. When I'm making bibs and the front fabric is dark and back fabric is light, I'll use 2 different colour threads so I don't have ugly black thread on a white fabric. If you adjust your tenison just right it works.
But for hemming grey pants, just use both colours the same :-)
You can use two different color threads - if I'm sewing something where the stitching won't be visible, I sometimes just leave the bobbin thread whatever color I last used so I don't have to wind a new bobbin!
Re: If you use a sewing machine
Ahhh! I see.
Do I use the same color thread for the bobbin and needle?
What if I were hemming say grey dress pants...
you can find some really great sewing tutorials online by just googling "sewing tutorial"
Or there are some great books at the library of Joanne's of all sorts of beginner books
hope that helps
You can use different colour needle and bobbin threads. When I'm making bibs and the front fabric is dark and back fabric is light, I'll use 2 different colour threads so I don't have ugly black thread on a white fabric. If you adjust your tenison just right it works.
But for hemming grey pants, just use both colours the same :-)