I pretty much always have a light to medium slate blue/grey and an off white to tan, one on top and one in bobbin loaded on my sewing machine and use them for almost all my piecing. I mostly scrap quilt and can rarely match thread more than a piece at a time, but slate and camel disappear most of the time.
This year actually I'm trying So Fine as a bobbin thread so it's all white on the bobbin.
I only buy "fancy" threads as needed for specific projects.