This digital multimeter is useful for evaluating textiles and thread for conductivity and electrical resistance.
Because each colored led have a different forward voltage different "scales are provided", Blue is good for low resistance,
Green for middle, and red for higher resistance.

It is is easily built by soldering the anode or cathode of the RGB led (the longest lead) to the appropriate battery terminal.
The other leads are twisted with round nosed jewelry pliers into spirals.