I prefer the belt system as it gives a very visual image of a students progress, our school has white yellow orange green blue brown black (with high and low to each colour). As for should kickboxing be considered a martial art or a martial sport, I think that would all depend on the mental aspect of your training, if you incorporate the full martial discipline in your everyday life as well as in your training then it becomes an art, if you just focus on fighting and techniques then it would be more of a sport.