I made a set of recolored diagonals to match my straight recolors so I have, at least, the ability to change the color of the diagonals in the game. I'm going to remove my diagonal recolors and try out your suggestion of using the eyedropper. That may work.
I have noticed a problem with your 1-tile doors. When I placed them on a wall in my test hood, there was a strange shadow on each side of the doors. The shadows are about the width of the door frame and positioned away from the side of the door about the width of a door frame. I don't see them in your screenshot though. Were you playing with shadows off perhaps when you took the screenshots? FYI, my test hood is in a a vanilla game.
Update: I tried your suggestion. I removed my diagonal recolors from my downloads folder, leaving only my straight door recolors and opened my test hood. I placed a 2-tile door on a straight wall, recolored it and used the eyedropper on it and placed the new door on a diagonal, It retained the recolor. Also, If I pick up a recolored door and move it to a diagonal wall, it retains the recolor. In both cases though, if I try to recolor the recolored door that I put/moved to the diagonal wall, only the 3 EA colors are available.