The general rule for Woodpile blocks is to have gradual transitions from one color to the next within a color scheme. Of course, you can break the rule if you have a special effect you want to try, or if you want to live dangerously. Page 9 and Page 10 in "Answers and Extras" give some special effects, good or bad, you might get if D strips (largest dark strips) are high contrast. See how the reds create a new design?
Page 10 of "Answers & Extras" gives more examples in black and white, plus references to color photos in Sizzling Quilts from a Simple Block which show other effects.
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