These are meant for short term approximate dosemeters.
Basically a guy has two dosemeters if he enters a high rad area. The film/TLD type that you need a reader for, and the tube type that he can get a ballpark reading for.
They do discharge easily. A hard rap, or just leaving it sit will take them down.
What you do is put the pen tube thing down over the light button. It fits over it. Look into the tube. You should see a gauge in the tube, and a line to indicate 0- whatever its high end is.
There should be a dial on the box, and it allows you to zero the pen. As high energy radiation hits the tube, it cause some charge to be lost, and that causes a defelction on the line so it shows the radiation you are getting.
Again it is only for a ball park reading so you know when to get out now.
My bet is the different colors are for the different ranges. One in Millirem, and the other in Rem.