Here are our finished projects from our trade. Sandy's entrelac bag is a the right, Pam's hexagonal throw below and my intarsia panel at the left.
We all agreed on three stock colors: Lanaset Scarlet, Washfast Acid Bright Orange, and Washfast Acid Ivy. Each of us added our own fourth color. I used Washfast Acid Brilliant Violet, Sandy used Lanaset Sun Yellow, and Pam used Washfast Acid Rose Pink.
Our three main colors were two analagous spectral hues plus a compliment. We didn't set out to chose these colors on a formula, but by happy accident, any colors we added to this mix would be harmonious, as the main colors anchor the whole. If you wanted to try this with different colors, chose two colors that lie beside each other on the color wheel, such as green and blue, violet and red, green and yellow, etc. Then chose the compliment of one of those hues. For instance, if you chose green and blue, your compliment could be red, orange, or for fun, red orange, a mixture of the two. Then chose a fourth color.
Once you chose your colors, you can mix them any way you like, dilute them etc. All of us just happened to start with a full strength version (FSV), then did a lighter version (LV) to match. Although the lighter versions didn't seem to go that well with the full strength ones in the fiber, just look at how lovely they are in this intarsia panel. The brilliant red based yarns are Sandy and my FSVs. The dark areas are Pam's. Warmer LVs are Sandy's, cooler ones mine. I saved Pam's LV for socks...couldn't part with it.
Pam groups most of her FSVs and LVs together. You can really see a lot of her LV in this throw. She connected her hexagons with black handspun which gives this quite a honeycomb effect.