A while back – couple of years ago? Can’t remember – I started using colour, especially when teaching in the early stages.
I use colouring pencils to show the structure of pieces. Like this, perhaps: (from Kabalevsky Op 39)
The phrase structure leaps out once the “matching bars” are coloured in. The pupil can see at once which bars are repeated, and where the “danger” lies.
I might use coloured lines to, for example, show every leap of an octave. Or to show which parts of a piece are soft, or loud. It just depends on what we are focussing on at the moment. In the tutor books, I (or the pupil) will colour straight onto to page; for more advanced students I’ll take a copy of the section we are working on and mark it up, rather than write indelibly all over the original.