A fungal toenail can look discolored (yellow, white, brown, green, black) and be thickened and even crumbly or brittle if a fungus is present. That being said, these color and appearance changes can be from other conditions as well. In order to know for sure, a nail biopsy is required for a pathologist to evaluate and perform testing on the nail to assess for fungus. In some cases a nail is simply damaged but without a fungal infection.