I used ultramarine, vermillion hue and olive green to make a greyish-blue which I used to color my cells.  I really like the effect!  I love how the colors tend to separate in some self-mixed colors.  I also really enjoyed this project.  I liked the (new-to-me) technique of painting around the edges for a dark outer edge and then blending gently toward the center, it gives a lot of depth I haven't been able to achieve with other paintings I've done.  And I have a special fondness for shapes that fit together so I think I will be exploring this theme in more of my art.