This may sound “picky” at first, but you must define “training” before you choose a color code. IF you are talking “practice” then the light blue is correct. However, in a lot of ordnance, this color does NOT denote a completely INERT or dummy item. Many items; practice mines, mine fuzes, bombs and SOME ARTILLERY PROJECTILES can have explosives inside, even though they are painted blue. That charge invariably will be a smaller charge of a less lethal or less “brisant” mixture, merely to indicate functioning or to allow observation of the function; however, it is NOT INERT. Although it is normally acceptable to “assume” a blue painted 20mm PROJECTILE is INERT, ie. no explosive charge whatsoever. BUT to get into a habit of doing this is incorrect–the casing is still hazardous as it may have propellent, the projectile may have a tracer element, and you may lead someone to think that what is true for 20mm is true for example, 155mm, and that could be actually hazardous. It is best to stick with the “correct” answer, and BLUE projectile is practice, not “training.” Initial training is normally done with totally inert items, often called “dummy” and I have seen these in unpainted steel, a “galvanized” look, and painted black. Currently there is an anodized bronze color for 20mm “inert” rounds, in addition to the above colors, and a couple of shades of “off-white” plastic.
Just trying to keep us safe, not be critical, esp. as Shotmeister had fully qualified his statement, and warned us already!