I started with some variegated green from my stash of vintage thread ("vintage" is code for "old stuff that's been kept around for no apparent reason"). This is about three-quarters of design #38 from Lavori Artistici 11, until I ran out of thread. I like the way the colors swirl.
Then I tried something a little bolder but still variegated for design #12, also from Lavori Artistici 11. Amazingly, this one won a first-place ribbon at the Western Washington Fair this year. Go figure.
Then I threw timidity to the wind and went for the "clown colors," one with a star-style pattern and one with a leaf-style with stacked double decreases. Both are from "Danish Lace Treasures" by Gloria Penning; one is Judith and one is Patricia. Not too bad. They might make fun sun-catchers.
As a result of these experiments, I think that variegated and multicolored thread can work for doilies with simple patterns. Would I do a Niebling with variegated or multicolored thread? No. But those who have supplies of vintage/old stuff thread can use it for doilies — and maybe even some crocheted edgings for hand towels.