I had always used Gimp for my huds. However, it's gotten more complicated for some reason to use. There most current version is not as intuitive. I may go back to an older version if I can find it.
I also have Corel PaintShop Photo Pro. I mainly use that for my phtography however and not huds. Adobe Photoshop would probably be better. I'd try Gimp first. I suppose you can get a trial version of Photoshop.
THEORY - you know everything but nothing works. PRACTICE - when everything works but don't know why. For me, theory and practice are combined: nothing works and I don't know why.