I have tried using a single colour of pen throughout my Filofax, but it just doesn't work for me. I like to see at a glance what is important (in red, of course), what is work (green), personal is in purple, and so on. I also highlight in different colours.
I used to colour code everything when I was back in school, but for me it just became another form of procrastination... I tend to write everything in my Filofax just using my fountain pen in one colour & only very occasionally may write something in one other colour to make it stand out.
I love to write in different colors. Most of the time it is not a problem: birthdays, activities of the boys. But I find it easier to write the regular appointments in black. Nice and easy.
I always write in the same colour, black. For important tasks l tend to circle them rather than use a different colour.