Now, brown in general makes blue eyes pop a little more than blacks and greys (as it is closer to orange which is the opposite of blue in the colourwheel), however the RIGHT shade of brown makes such a diefference! The right shade for me has proven to be Embark. It has that red undertone that makes the blue in my eyes pop more without being a red or an orange colour. It defines my eyes and makes them look brighter. So what's not to love??
I took a picture of my eyes, one wearing Embark and one Mystery (a cooler matte brown- grey undertones) so you guys could see the difference between hem. Though the effect didn't show up as much as I would like it to (as it actually does in real life). You can still se it though =)
Embark VS Mystery
If you hold your hand over one eye and then the other you can see more of a difference but if you look closely you can actually see that the left eye looks slightly more blue and the right eye looks slightly more grey. So imaine the effect in real life (when the colours aren't that much washed out - the flash does that)!
So conclution? I love Embark as my everyday defining eyeshadow and I suggest everyone with blue/grey/green eyes to go and check it out. If it's too exspensive maybe go check if the drugstore brand have a similar redtoned matte brown. I bet NYX has one, and maybe you have one in a 88/120 palette that you own?
If you have brown eyes though a more greytoned brown would make them look slightly more brown (grey is closer to blue).