Grobs, Fireflys, Extra 300s, can be painted/resin impregnated any colour you like - yes there are issues with black on upper surfaces but all EFT Fireflies went from white to black undersides many years ago with NO structural impact.
The issue is cost and time out of service. In adition, if re-painted after production there may be a weight penalty as there was with the T67M160 early models.