I want to share with you a system I’m using.
I have 100 objects for 00-99 numbers (no matter how you form them, Mnenonic Major System or something else).
To extend the system to 4-digit I used colors.
07 is socks for me. In colors, 6 is red, 3 is white, 1 is black (you form own colors from 0 to 9).
Background goes first. Foreground is second.
Black Socks with white text is 1307.
Red Socks with white is 6307.
For Spanish, Italian and French people(Romance languages), color can be used after an object, i.e. red socks with white is 0763, and black socks with white text is 0713.
Also, when I color an object, I imagine material(color) of the objects.
If we add 100 categories for 2-digit objects, we can extend the system to 6-digit.
That’s all. So, what do you think about it?