They don’t have any special meaning. I’m using GeoMagic Design to design the machine and drawing the illustrations. I like that software very much, but there are some shortcomings. One of them is that the part trail color is not user-selectable. Some cases the default white works fine, some cases it doesn’t. When it doesn’t, I draw green trails manually.
Btw, the illustrations are screenshots, where I have manually added the balloons. The screenshot utility I’m using is called Greenshot; highly recommended! The green part trails and number balloons are drawn in Greenshot as well.
(I have no association with either of the companies, just a satisfied customer.)