Thanks Dan, It turns out that this wasn't the actual problem. I was getting a javascript error that made it seem like setColorAt was getting a null object. It turned out that I was hitting an object limit. Once I simplified some of my classes to maintain a spritesheet rather than a number of arrays of GreenfootImage, it eliminated the problem. This is the scenario I'm working on, if you're interested: https://www.greenfoot.org/scenarios/25314

I like that you took the initiative to put this up on the Gallery! I know it took some extra work. Thanks! I'm going to use this as an exemplar for future years :)

This is a great asset! I was going to make a weekend project out of creating my own, but this does an excellent job - very modular, well-commented and useful. Thanks!!