A nice bright yellow geometric shape will stand out well. A few that come to mind is a star, circle, 3 inch wide stripes of yellow running from the wings leading edge back towards the airleron.

The stripes would be easiest. Just by the same type of covering you used to cover it with, then cut out the stripes, spray a light mixture of soapy water or plain old windex on the bottom of the wing, place the stripe on it, then use either a piece of soft balsa or cardboard to act as a squeegy and smooth out the stripe. Lay down all the strips the same way. Once done, take a paper towel and remove the excess liquid carefully. Then let it all dry overnight.

When you use the soapy water or windex, this allows you to position what you are adding without it sticking right away