Apologies if this belongs in IMHO …
I think every pair of blue jeans that I own is sewn with yellow-orange thread. Is there a reason why denim is usually sewn with a contrasting color?
I’m asking because I bought some indigo denim last week and I was going to sew a pair of shorts, but I have no idea if I should use yellow or blue thread.
Depends if you’re asking:
a) Why contrasting thread colours are used?
or
b) Whether yellow or blue thread would be better.
One of these belongs in GQ, the other in IMHO.
I think it started out because jeans have that heavy welted seam, so it had to be sewed with heavyweight cotton thread, which happened to be yellow. Regular weight fine sewing thread in a matching color wouldn’t have been sturdy enough.
If the shorts you’re sewing have a welted seam, I’d say use a heavy yellow cotton thread, so they’ll look like jeans shorts and the crotch won’t bust out right away. But if they’re just plain summer shorts, don’t bother.
Also, it depends on what you want it to look like. Using matching thread, IMO, will make it look more “homemade” and less like Wal-Mart.
Am I missing something here? Couldn’t they have used heavy weight thread in a matching color?
According to uselessknowledge.com, the thread was selected to match the rivets in blue jeans. Unless you’re planning to rivet your shorts together, I’d use the blue stuff.
Yellow thread is sort of a “standard” when sewing jeans and denim. Though I have seen garments with indigo thread used on most seams, with the heavy yellow thread used on certain “accent” areas of the garment.
I have sewn lots of denim (or heavy twills) and I like to use a contrasting thread at times. Just for fun. (On a pair of black jeans, I used fuscia thread, on a pair of indigo jeans, I used teal thread. Yeah, I’m a real rebel.) I don’t think people guess that these garments are hand-made. (They always seem surprised when I tell them.)
DuckDuckGoose is right about the crotch seam - reinforce that seam area a lot (A LOT) so it won’t bust out. Also, denim ravels like hell - finish your seams extra well. (I use a straight stitch and then serpentine stitch over each seam to finish.) Yes, yes, more information than you ever asked for. I just like to ramble about sewing!
Thanks for the answers, everyone. I was going to use blue thread, but I wanted to find out first why most jeans are sewn with yellow. I didn’t want to put all that work into making the shorts and then have them come out funny looking.
And to Duck Duck Goose and yosemitebabe, thanks for the added sewing advice. 
It’s a lightweight denim so I think regular thread will be fine, but I’ll make sure I really reinforce those seams anyway. (BTW: I love the idea of using fuscia or teal as contrast. I’ll have to try that.)
Just to confirm what ENugent said, I looked up Cecil’s column on Levi’s jeans