Yeah, I'm continually impressed by how cheap sensors are in SR. Every time someone says, "Oh yeah btw these cost like a weeks allowance money." it throws me for a loop.
I'll probably get a geiger watch with a barometer (Which are surprisingly expensive, but an atmosphere sensor that I would assume uses barometric, is cheaper?) after reading through all this.
I would assume that the badge just gets darker as it's exposed, but it might use some sort of scintillating chemical reaction and a variety of filters to turn different iridescent colours depending on the type of radiation.