If the hole is tiny and the fiber feltable, you may be able to use a needle felting tool to felt it closed. Otw, you could try Swiss darning (youtu.be/f-SJEwSP4HE?...; knitty.com/ISSUEspring0...). It's harder w/ machine-knit fabrics because of how thin the yarn usually is, but I've had good success

The Swiss darning video shows a repair with contrasting yarn, but if you find a yarn that is a close match in color and weight, the repair would be much less noticeable. Another video that may help: youtu.be/soZoUyV7utw?...