Perhaps yellow , green , then brightgreen for sandbox , incubating , and graduated?
After thinking more than a second, I realized they were too close in color and could be difficult for people to see the difference, especially vision impaired
I took a screenshot of the supported colors from shields.io and ran it through this tool: Coblis — Color Blindness Simulator – Colblindor
It seems that lightgrey, blue, and green/brightgreen provide the best contrasts for different color-blindness.
