I presume it is because color will be used dynamically to denote when something goes wrong. A mater of "contrast" if you will.

"The nodes also can change color based on assumed health of the underlying service to give another quick focal point for where problems might exist in the system."

See this screen:

https://4.bp.blogspot.com/-lFqeDOUmRhA/V5-dtXr1HQI/AAAAAAAAA...