Actually stretch marks don't tan like regular skin, you can use a spray on/lotion tanner on them.
Some women have also reported that using a self-tanner helps to camouflage their stretch marks. If youï؟½re going this route, remember that it is specifically tanners that you apply yourself that work. Since stretch marks donï؟½t really tan, sitting out in the sun or lying in a tanning bed for a while may make your stretch marks more noticeable as they remain light but your skin gets darker. Besides, tanning in the sun and on a tanning bed exposes you to UVA and UVB rays, which work to damage your skin, increase your risk of skin cancer, and can cause premature aging. So reach for that bottled tan instead.