Diligent
Careful, Elaborate, Meticulous
noun, な adjective
If you don't want your thoughts to turn rust colored, you need to be diligent and studying, learning, and experiencing new things. Without diligence your brain and all your thoughts will get old, and stale, turning a rusty, gross color.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
熟練の染師が、一つ一つ丹念に染め上げました。
The skilled dyeing master diligently finished dyeing each one.