Research shows that when we colour co-ordinate our learning it improves our memory.
So... how are you encouraging your learners to use highlighter pens?
Points for an argument in green, points against in red for example.
Feminine words in red, masculine words in blue.
Integers always in yellow.
Or carnivores in red, herbivores in green and ominvores in red and green stripes (or, as one year ten lad told us, omnivores in all different colours because they eat all different things).