Try A Root Touch-Up Pen
Total Beauty mentions this hack as being a great one for covering up minimal grays. Touch-up pens are like a cross between mascara and a Sharpie but built for hair. That sounds kid-friendly enough, right? Super simple and you probably already have the materials lying around.
Total Beauty mentions that this is best for thinner hair, as it won’t weigh down the hair or color your scalp (which is a risk with a spray or dry shampoo). While there are many brands out there, experiment with a few to find your favorite applicator size and color combination.