The way it works at the moment is that if there is no mag inserted, then it says INSERT. This is detected by a switch, once inserted it detects the mag size the same way as my other one, using hall sensors and micro magnets, and then shows mag size and rnds remaining. The rnds remaining assumes you inserted a full mag.
Then the shot counting works the same as before, using a switch or hall sensor on the trigger, which will depend on the gun. Once it reaches 00 it flashes the reload sign.
Its currently setup for battery supply V and A draw, but i might make that just the voltage, and add an RPM indicator like you suggested. I'd do that via an IR sensor on one of the flywheels. Not sure how fast it could update the display though, probably only once or twice/second max.
For your plan, an RGB led is easy to drive. For mag insertion detection in my stryfe and rapidstrike i just re-purposed one of the existing switches used as part of the locking mechanism, but the Centurion may not have that available. A hall effect sensor would be easier than a reed switch though, they're smaller, and dont need as large a magnet to operate. The magnets i'm using are only 1mm thick, 3mm round disk magnets that can fit anywhere. Cost me about $3 for 50 on ebay.
To use a reed switch you could use a digital or analog input, detecting when it closes in the presence of a magnet, completing the circuit and driving the pin high or low as you prefer. I'd go low, as you can set the pinmode on the arduino to use its internal pullup resistors. A hall effect sensor would only work on an analog input, as it outputs a voltage relative to the magnetic field. With a 5V supply, it outputs 2.5V with no magnet present. In the presence of a north or south magnetic field, the voltage either drops, or rises. That way it you can detect even a weak field, and dont need a full contact like a reed switch. It also lets you do two detections from one input. So a mag marked with a north magnet may be a 6 shot. A mag marked with a south magnet might be a 12.
You'd definitely need IR to check that a dart is chambered though, thats something i hadnt considered doing. I had thought about detecting how many rounds are actually in a magazine, but i'd need to modify every magazine for it.