Tips for faster sewing - Melly Sews - Use Gray Thread for Inside Seams. If it’s not couture, it’s unlikely that people are going to see and/or care about what color thread you used on the inside of things. So go with gray. It blends into most everything.
[FABRIC + THREAD + NEEDLE CHART] Thread advisor, shows fabric type, thread to use, and needle to use. Site also has pdf file version. Good chart for hand & machine needles but only specifies one brand of thread--that of the advertiser.