Eye Dropper tool not working

Without any info about your system, your version of AE, your project, what color you are trying to sample from where and so on we can't realyl tell you much. Most likely this is one of those gigazillion hardware acceleration issues or it's realted to your screen configuration and e.g. problems with color profiles. Start by checking those things and update your graphics driver if necessary. Anything more will require proper details.

Mylenium