I've never seen them on bikes before, but I have seen them on tricked out import cars. I've seen them for sale at tuner shops, and if I remember correctly, they has them in multiple color options. Would definitely be an attention-getter.
I just called a good friend that is in Law enforcement and asked him if these were legal in Pa and his answer was "Nope, shouldn’t pass inspections, called ornamental lighting" So there a moot point for me.
You need to be careful and check the clearance of the stem once installed . It could hit something like the brake caliper, tear the valve stem and cause you to get a flat out on the road . I saw that happen to a friends Harley with bullet style valve caps . Jeff .