I used colour in my images many years ago to represent digits. What has changed over time is my palette for representing digits. The more colours I put on the palette, the more two colours or shades will look similar - and that is undesirable. There are three shades of brown in the palette picture below - and they are quite close to each other in tint!
By having this idea that a colour can indicate a digit (or represent a letter of the alphabet), it opens up the possibility of memorising information by remembering the colours of a picture.
I intend to use this feature in many of the images which I home-make. The colour of someone's hat or clothing top or trousers or hair can all mean something: digits or perhaps letters of the alphabet.
I would like to have a suite of games where the characters are introduced and learned by students - and then, later, memory games can be introduced to the student based on the colours which comprise them and any icons on their clothing.
The characters in the earlier lesson for representing digits 0 to 9 have had their colours chosen based on the palette.