Hi! When using any external monitor on a macbook, I'm having trouble using the "Pick color from screen" feature. For example, the color #888888 becomes #939393 when using this function. There is no such problem on the native MacBook monitor.
That's because your external monitor is not calibrated to show the colours in the same way as your main monitor - but it's impossible to do so from my own experience. If you use Apple's Digital Color Meter to sample the same colour on your two monitors you'll get the different results - i.e. it's not an XD issue.