I'd use a pen too.

Try a test sample to see if you can match the red with a marker. If it was black, brown, blue...the darker colours you'd have no trouble getting a match. Other colours can look worse if they don't match.

I've done it with black, It'll even go through the wash if its permanant!

just leave it the way it is.

Never thought of using a pen! Have to remember that! Small mistakes no big deal!!

I say get a few markers and practice. Sally Collins has approved this for stray threads showing and she is wicked fussy, so it should be fine.
please show us how it came out!

Me too - I use colored sharpies to fix those little mistakes. (And it works great!)

I've done this before with quilting and embroidery. Works fine.

It's not a big area at all. I'd pick it out and redo, personally.

Great idea!