What is the best way to design voltage monitor with tri-colour LED?
OK, thanks for the tip on using the bargraph meter chip, I've persevered and come up with this:   



I found this article on Nuts and Volts which gives details on using the LM3914 and getting the best out of it:  http://www.nutsvolts.com/magazine/article/led-graph-circuits.

The circuit works pretty well, the sensitivity can be adjusted by varying the 'window' set with the trimmers R2 &amp; R3, and also by choosing different LED outputs.  The green 'normal' LED needs to be on LED_5, pin 14.