How They Work

Healthy plants will look dark using the purple filter because they are not reflecting red or blue light. They are health and photosynthesizing fully, absorbing all the available red and blue light.

A healthy looking looks dark when viewed through a purple filter
Stressed plants will look lighter using the purple filter because they are reflecting some red and blue light. They are unhealthy and not photosynthesizing optimally, therefore not all available red and blue light is being absorbed.
A stressed plant looks lighter when viewed with a purple filter because they reflect more red and blue light