I had dinner with some friends the other night. One of my pals asked me about wearing socks with flats...my immediate response was no! That would look crazy!
As I thought about it, I think need to qualify my response...
First, if you are under 20 - socks with flats are fine. I would take issue with an 8 year old walking around in the bitter cold of winter with no socks on!
Tights are always appropriate with ballet flats - regardless of age.
A think trouser sock would be fine with ballet flats. Just be careful of the color. You don't want the noticeable thing about your outfit to be your socks!
I have several trouser socks that are nude or skin toned. These are the socks I wear with those cute shoes that I don't know what socks to wear with. I got them at Target. Give that a try!