The caps can be used with any air or nitrogen you put in your tires. If the tires had nitrogen you wouldn't be losing air, so bobrussel's comment about the advantage is pretty correct (the advantage is not as good as we'd like).
The caps can be used on any tire. If you see green DON'T do anything. It's only when you see red that you need to fill the tires. These caps give you an immediate indication when you need air, instead of having to use a gauge - their advantage is that a gauge can cause air loss when taking your measurement and these caps eliminate that inconvenience.
You will need to know what the rating on the cap is supposed to be so that when you NEED air you won't overfill. There are different caps for different tires and hopefully you have a properly rated cap.
You can mix air and nitrogen-enhanced air. Just know that as the ratio of regular air rises, you'll lose more air over time which is what nitrogen helps prevent.
Until Nitrogen is more readily available and free, I'll keep using regular air.
The valve stems covers (caps ) are just that, there is no indication of tire pressure. Instead of the typical black cap on valve stem his are green. The green stems ussually indiactes thet the tire was filled with nitrogen. Most people will just top off with air if pressure is low becuase it is free and can be found at most gas stations. The ones Artmart is talking about are clear colored stems with a green indicator for normal pressure and red for when tire is low.