It's unfortunate that not every diagram contains the legend from the first diagram in the GitHub repo [0] which explains yellow as "Personal recommendation" and orange as "Available Options". I think grey was "Optional".

[0] https://github.com/kamranahmedse/developer-roadmap

Yellow means "Personal Recommendations" and the peach one means available options. This will be more clarified/fixed in the next release.

Good default, general industry consensus for best option if you have no particular needs.