I know one, it's called Blockbench and it's a lot easy to work with, also it's available for Linux and Mac, you should try it out

I mean, I know how to get the texture and change certain pixels, but I don't understand how the texture file lines up with the 3d model. I just have my trident mostly purplish-black, but I want one light blue spot in the middle thing (where you have one), and the back tip up to three pixels similar. Here's a picture.
[url=blob:chrome-untrusted://media-app/8b7bb8e9-e6f6-4cde-b33b-a40bf48ab962]Web Address of the picture (safe i promise)[/url]
i can give you my file and, if you're alright with it, you could put the center piece ther yourself with your program, bu if I knew how to do it myself, it would be recreatable.