Based on http://en.wikipedia.org/wiki/Grue_and_bleen, concept of "blue" is not homogenous, and some may prefer "green" as greenballs plugin provide, but some may prefer some other color.
Suggested improvement is to let jenkins admin define the balls color based on a hue selector, and dynamically generate the image file.

Implementation notice : the svg for blue ball can be used to generate an image (as jenkins does during build), but let user tweak the gradient colors and generate custom variation for bleu/green, by select a point in http://en.wikipedia.org/wiki/RGB_color_space with only Green + Blue axis (I don't think red would make sense here)