What is the optimal combination of text and background color for EEG experiments? Most studies that I know of use yellow text on blue background (though with different hues). 

This choice is mostly motivated by tradition ("We always did it like that. Never touch a running system"). But is this combination really the best one? 

I have found a number of studies about color combinations and readability. Some suggest that black text on white background is the best, others say white on blue, etc. 

I imagine that there are specific requirements for EEG studies as they are regularly run in dim lit rooms. So a completely bright white screen maybe not the best idea, I think. 

It surely does make a difference if the task is dependent on, e.g., quick word recognition or reading a short story. 

And what about flat screens vs. CRT screens? Are there different optimal color combinations for these screen types?

Is there any general optimal combination?
Are you trying to measure something specific to the stimulus you are presenting or is it just a matter of displaying text on a screen where what you are measuring is unrelated to the text per se?

For instance, experiments I run depend on the stimulus presentation. We are targeting a certain visual pathway, so for example one stimulus is red/green and high contrast, the other black/white and low contrast, both on a black background. We have also run on a grey background as that can be easier for the eyes. And we also have dim lit rooms, to help amplify the signal or even lights off. Also, the stimuli are flickering at a high rate so we use a LCD monitor with a refresh rate of 120hz. We used to use CRT monitors though as the black color is blacker there then newer monitors.

It just depends on why you need to optimize it. Is it for a true signal you are recording? Or simply comfort and ease for the participant?

If it is simply for comfort, I would recommend no color, but grey background and black letters, and dimmed lights. If the contrast is too high, such as black background and white letters, this could be a strain to read over time. There is no gold standard. It's according to what your needs are.