Most projects that use many colors use one main color to tie the others together, especially "stash buster" projects, where the other colors might not coordinate well.
Often the color used to tie everything together is a fairly neutral one: black, brown, navy blue, white, off white, grey. However, when the colors you are trying to tie together are floral, using green or a camo-themed variegated yarn as the theme color can give the impression of an abstract flower garden.
In working on blankets for charity this year, I made a baby blanket using up leftover bits of pinks and purples that I tied together with olive green. In the past, I've also created the impression of roses by using dark red and camo yarn (leftover from making sweaters for my sons).
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