|I've done alterations for some time, but few places I've worked had a wide range of of colors in heavy jeans threads.
I found a way to cheat a little. I wind a bobbin with a double thickness of thread, winding from two spools at once. When stitched, it looks like heavy-duty thread. I have even used two colors together to blend a color I don't have.
I use regular thread in the needle, which ends up on the inside of the hem.