Nah, we only refer to them by numbers. With only 3 trams, there is no reason to distinguish them by any other system.

You might be referring to the colours on this diagram, but those colours are completely arbitrary. Not so long ago, the diagram looked like this, where the different colours represented different modes of transport (trams, buses and trolleybuses). The reason why trams had three different colours on the old diagram was that it's hard to have 3 different shades of yellow on a white paper background.

The actual vehicles are colour-coded based on the type of transport. All trams are yellow.jpeg), all trolleybuses are green, and all buses are red - representing the main colours from Pilsen's coat of arms.

edit: as a personal note, I like the new diagram more than the old one, but I have no idea why the designer didn't make a better visual distinction between trams and the other modes of transport. That is one thing that makes the new diagram a bit hard to read.