I've done a bit of searching on Google, Martin, and found that first aid boxes are available in green, red, white (with a white cross on a red circle) and blue. Looking at some pictures I downloaded some time ago of German paramedic vehicles, they have silver, white, orange and red cases and bags. On British paramedic vehicles I've seen green, red and blue bags in use. Overall, I think you can devise your own rules, therefore.
In the small cases, Playmobil offer red, blue, white and silver and there are two sets of equipment stickers for them (see attachment). My own system for ambulances, including the air ambulance, and my fire vehicles is:
Red case - defibrillator sticker in the body of the case (I don't know how familiar you are with the cases, but there is a standard insert to take the stickers), with a sticker of wipes, gloves and bandages in the lid, thus creating a case with dual storage.
White case - white oxygen cylinder attached to case handle with wipes, gloves and bandages sticker inside case lid and oxygen tube with face mask in case body.
Silver case - drugs sticker in case body and wipes etc sticker in case lid.
All of these can be carried on the ambulance and air ambulance (and the custom 4x4 support vehicle for the air ambulance I'm working on). The red case is carried on all fire vehicles but the silver case can only be carried on those which always carry an EMT trained crew member. At the moment, that's only one of these, the jeep control unit. Thought is being given to whether all front-line fire vehicles should also carry the white case and oxygen cylinder to allow oxygen therapy treatment for smoke inhalation (the BA cylinders only contain compressed air and so are unsuitable for this). I suspect that the end result will be the jeep will have the drugs case replaced by a defibrillator as being more useful but no oxygen cylinder and case.
Looking at the attachments, the top one is from the new ambulance, 4221, and is part number 30 88 1292, and the bottom one from the 3925 ambulance is 30 89 3280.