Most commonly, yellow or brown discoloration to a toenail is due to a fungal nail caused by a fungus call Trichophyton Rubrum. This fungus gets underneath the nail and also commonly causes thickening and/or crumbling or brittleness to the nails. The discoloration can be something underneath the nail as well, such as skin cancer. To know for sure, an in office test is usually performed. The longer you wait to be evaluated, the harder a fungal nail can be to treat, so don't delay!