We can't know. You have not provided even a shred of useful info like version of AE, system info, footage used, comp settings and whatnot.
Are you running CS5.5 on Mac OSX 10.10 or 10.11? That often has crashes because CS 5.5 is really old and has weird bugs on new Mac OS's. You could try changing your "use system color picker" in your AE preferences.
If you're not running CS 5.5 on a new Mac OS, perhaps some more information could be forthcoming? FAQ: What information should I provide?