Is the colorspace R package part of the Tidyverse? Looking at this list of Tidyverse included packages it appears the answer is "No".
However, when I load the tidyverse via library(tidyverse) it appears that colorspace is loaded. I can request help with ?colorspace_function() and the functions appear in my R Studio help window. If I start a new session of R, don't load any packages and run ?colorspace_function() I get a "no method" error, as expected, indicating the package containing this function isn't loaded.
I want to make sure we distinguish between "loaded" and "attached". Yes, colorspace is loaded, but it is not attached.
I can replicate what you describe, loading tidyverse and then successfully using ?choose_color, but if I try to run choose_color() I get a function not found error.
Looking at sessionInfo(), colorspace is listed as "loaded via a namespace but not attached". So probably some package in tidyverse imports a package that imports a package that imports colorspace, and RStudio lets you follow the chain to get to a help page.
You can even play a game to try to get from the tidyverse CRAN page to the colorspace CRAN page by clicking only on "imports" and "depends" links. I did it with tidyverse > ggplot2 > scales > munsell > colorspace. (Or you could write a script to trace the dependency tree.)