Bogu, kendo gi and hakama are often made using Japanese traditional indigo dyeing technique called "Ai-Zome" (藍染).
Do you know the history behind it? Initially, the very first bogu invented had no dye; it was a white bogu.
Ai-zome became familiar with common people from the Edo Period. Ai-zome became popular among samurais, and it was used on bogu as well. Now, it is indispensable for kendoka.
Why did the indigo dye become popular among samurais? As the indigo dye starts fading, each stages have a name for it.
In one of the stage, there is a colour called "Kachi Iro". The word "Kachi" is a homophone to "win" (勝ち). Back then, samurais favoured any superstition for good luck/fortune. Therefore, they favoured the indigo dye for having "Kachi Iro".