0. 44209. Rash on scrotum causes changes in the skin of the testicles thus affecting its appearance, texture or color. There are several reasons that would cause a rash to appear on the testicles sac, inner thighs and the penile shaft. Common causes of these rash will include STDs, candidiasis, heat rash, dermatitis, eczema, and scabies among. Rub the ash onto your skin, and keep rubbing gently for a few minutes. Once the color is gone, use warm water and soap to clean the area. Keep in mind after the process, and you should wipe it dry to confirm that most, if not all, of the dye colors, are gone. Method 5: Nail polish remover.