My Walmart has lots of Sharpie colours that are $1 each and I get them for colouring threads or fabric flaws. I have even used one to colour in a paper piecing boo boo. Why is the offending tiny wrong coloured piece always where you can't fix it without completely taking the block apart?
I pick up my graph paper while it is on sale too. You could pick up white chalk for marking quilting lines too.