Contrasting thread is easier to do and easier to redo
Why visible
You can see your own stitches as you work, which makes the tension even. Even tension is what makes a darn last.
The technique
Weave over and under across the hole, then across again at right angles. Start well outside the thin area rather than at the edge of the hole.
The thread
Something similar in weight to the fabric. Thread heavier than the cloth pulls through and makes a new hole beside the old one.
