Buying thread for various projects or buying thread for specific projects

Hello fellow embroiderers!!! I am getting back into embroidery after a hiatus but I don’t want to have a surplus of thread I won’t use.

Just wanted to gauge how you all buy thread:

1. Do you buy thread for specific projects and then build your thread collection that way?
2. Or do you buy a set of colors and design your patterns that way?

Some tips would be appreciated. I would like to start on a project for my niece soon since her birthday is in March.