For annotating and adding a hand drawn signature to PDFs, preview really is the best: lightweight, straightforward, free, comes with the OS. I don’t know any comparable app for Linux (or windows although I rarely use it)
It’s local only (the document you load and data you fill in never leave the browser) and free
Disclosure: I’m the developer behind it
For other colors, it’s in the backlog!
(I’d aiming to automatically provide the correct color by inspecting pixels within the area to redact, to make it as simple as possible)