6 is blue, easy ref back to 4 red & 2 black. Same way, 5 also blue. 12 blue, ref back to 10 red, 8 black. Most of us know the lower #'s easily. Helps more when you hit the 20's, 30's & up. 36 blue ref back to 34 red and 32 black. 35 naturally blue too. 42 blue, 40 red, 38 black.
Same thing with brown, orange and yellow.
I worked a few jobs where we had setups such as Panel 1, ckts to 42, Panel 2, ckts to 84, etc. The 6's are handy the higher the #'s are, ones we don't use as much day to day.
Maybe this will will help out.
We all can remember on a 3? systems
1-Black, 3-Red, 5-Blue.
2-Black, 4-Red, 6-Blue.
Let's call these numbers "base numbers".
The same goes for Brown-Orange-Yellow, or any other repeatable color combination.
Just remember multiples of 6. 6,12,18,24,30,36 & 42. Let's call these numbers the "multiplier numbers".
Example:
You take the circuit number. You then subtract the largest multiplier that can be subtracted from your circuit number. It will be one of the following "multiplier numbers" (6,12,18,24,30,36 or 42). You end up with a remaining number. This number is one of the base numbers (1,2,3,4,5 or 0).
Match the found base number with the color of your system.
Example #1
What color is circuit #19 in a 208/120v system?
19-18=1 1 is Black.
What color is circuit #21 in a 208/120v system?
21-18=3 3 is Red.
What color is circuit #23 in a 208/120v system?
23-18=5 5 is Blue.
What color is circuit #20 in a 208/120v system?
20-18=2 2 is Black.
What color is circuit #22 in a 208/120v system?
22-18=2 4 is Red.
What color is circuit #24 in a 208/120v system?
24-18=0 If your number divides evenly with no remainder it is always Blue.
I personally carry a small laminated "Cheat Sheet" in my wallet. Black, Red, Blue on one side and Brown, Orange, Yellow on the other.