If you have a set of metric wrenches and sockets, let me tell you about my simple "peace of mind" colour code system.
Step 1: Five Rolls of Tape With Five Colours Are Best.
The colour code makes it easy to pick out all the same size size sockets and wrenches when you working on a car, motorcycle, bicycle or wheelchair. After you know one wrench or socket size needed for an engine compartment fastener, colour coded wrenches make it easy to pick up the same size wrench and sockets for other fasteners.
Here is a counting method to remember what colour matches the size of wrench you need. Hold up your hand, stick out your thumb and start finger counting "Red 10, Orange 11, Yellow 12, Green 13, Blue 14"
Recommended items: Use electrician's colour code electrical tape for marking. I bought a small marking tape kit that had only four useful colours. To make five rolls with easy to remember names, I bought a roll of orange electrician's tape. Five colours is better than four colours because metric wrenches are numbered in base 10 . You can see in the photo above the tape on the 12 mm socket is damaged. I am puzzled by this, as I try to be gentle when working on engines. I have begun putting two tape markers on some wrenches to make them easier to see and to accommodate the occasional scrape that removes a marker.
As the photo shows, I use scissors to cut 2 inch strips of coding tape.
If you find the word "colour" annoying, I agree. The spelling checker made me do it.
Step 2: Colour to Wrench Size Table
Wrench and socket colour code
0 Red 5 mm 10 mm 15 mm 20 mm Red
1 Orange 6 mm 11 mm 16 mm 21 mm Orange
2 Yellow 7 mm 12 mm 17 mm 22 mm Yellow
3 Green 8 mm 13 mm 18 mm 23 mm Green
4 Blue 9 mm 14 mm 19 mm 24 mm Blue