Bright sunlight will discolor anything, including linoleum floors. If you have applied wax to the linoleum floor, it may be the wax, and not the linoleum, that has become increasingly discolored. In the same way that wax becomes discolored from dirt and traffic, sunlight can deteriorate its surface. What is required is periodic cleaning and rewaxing. Use a strong detergent to clean the floor thoroughly, and then apply a wax specifically designed for linoleum. This should help your immediate problem, but you will have to put in a little elbow grease from time to time.