As Mike alluded to, yellows are highly variable in appearance. Actually, I believe that E. notaeus is one of the most variable in pattern and color of all boid species. I've seen dark ones as you have here, and others that are so pale that the pattern is barely visible, and all variations in between. One reason for this is probably the immense geographical range over which this species is found and the different types of wetland environments which it inhabits.

Kelly