Here is another little annoying problem I came across whilst in work designing in Illustrator. Every colour I picked came out a shade of grey! However, fear not! The solution is simple and you won't have to write any angry letters to Adobe!
Big thanks to Jay Hilgert for this one!
It's so simple:
1. Go to your colours window options (top right of the window)
2, Select RGB or CMYK (depending if you're designing for screen or print) and voilà!!
In that menu you'll see that it somehow selected "greyscale" instead. Very annoying and has happened to me more than once.