You said you found a trillium at the Skaneateles Conservation Area that you had never seen before. Thank you for sending the picture along with your question, it really helps! When leaves on a plant start to turn this yellowish color, it is called chlorosis. Chlorosis means that there isn’t enough chlorophyll in the leaves, which can be caused by a number of factors. Poor drainage, high alkalinity in the soil, nutrient deficiencies, and over or under watering. Because the veins on this trillium are green with yellow in between, I would say they are this color due to deficiency in a nutrient like iron. The trillium will probably be fine though, it looks healthy otherwise.