Alyssa, Owner of Annabel Bleu, always recommends mixing Small or Solid, Medium and Large scale patterns together. The large scale pattern is your "Anchor pillow"- accessorize the room around these colors.
Then, add visual interest by picking different sizes and textures for your pillows- Velvet, Faux Leather, Linen, Tassels, Pom poms, etc.
Finally- Do what looks good to the eye and fits your style, perfection is boring!