TIL that the colored section of a toothbrush is termed "indicator bristles." They signal when it's time to replace your toothbrush as the color diminishes.

https://www.westgatedentallincoln.com/our-blog/how-often-should-you-change-your-toothbrush#:~:text=Toothbrush%20Indicators&text=As%20you%20brush%20your%20teeth,differently%20than%20a%20manual%20brush.

https://www.reddit.com/r/todayilearned/comments/162isjc/til_that_the_colored_section_of_a_toothbrush_is/