What can I substitute turmeric for in a butternut squash soup recipe?

Turmeric is a root in the ginger family, not sure why you'd need to add turmeric for color in butternut squash soup, so I would use a small amount of fresh ginger (besides, ginger curried squash soup is so so yummy!).

turmeric by itself primarily provides color, you can get a milder version by adding saffron

Also, try roasting the squash to get a nice flavor . Peel and cut into cubes (size of sugar cubes) and coat with a hint of olive oil and roast until they start to caramelize. Then put into your soup stock and blenderize to desired smoothness.

Most curry powders or garam Marsala has turmeric in it ... if you don't mind the hint of curry flavor. You can also get a nice yellow color with saffron - but that can be costly ... but you only need a little.