Changing thick lines to thin

[Steve Austin]

I appreciate the reply, SallyAnne. Every time I try to do a magic wand and delete, you are right, the greys stay behind.
If I try to "add" back in some black with bucket fill, then I'm right back to where I was.
I am actually trying to make this into a coloring page. I have figured out a process to remove
all the color, but haven't figured out how to thin those thick black lines.