Seasaw2mch has the same idea that I was thinking. Lay the pieces of the block that you already have on top of different color choices to preview what it would look like. If you post pictures of the previews, I'm sure people on this board will be glad to help you decide which of the colors looks the best. Although you'll get many different responses!

I do believe the art principal would work. Dark, medium. light. the fabric should have contrast. don't put two fabrics of the same value next to each other

I think they are all well put my friend ,but maybe a bright gold yellow to make those sm triangles pop out more in the block

I'm also thinking a rich buttery yellow gold cuz your other colors are rich jewel tones and something less deep would perhaps seem washed out.

I see a lavender color in the black fabric that might be just right next to the green, and to the others. I love the way the lavender fits with the other colors in your dummy block.

In the red fabric there is an element of the design with what (on my monitor) looks a gold/orange dot pattern (around the berry bit with the leaves) maybe that could be a good choice for the little triangles.

I'd try some yellows in there to find which one I liked best.

Whatever you decide Don it will be a great quilt (as always yours are!)

I think it needs to be a darker color, maybe a blue or green, purple maybe even an rust. The light purple is definitely out

I like your first choice, the dummy block.