Elle...I'm using a varigated beige/camel/brown thread (King Tut) by Superior for the inside of my blocks and tris. I'm planning to use the same thread only in a darker brown for my between tris (which will be dark brown). I'll probably use a mono for my QITD when I put the back on if I machine that part. If I do it by hand, I'll use the darker thread for that part.
If you are happy with the white, I'd stick with that.