experimental design on matlab

hi,

I wanted to test the hypothesis that human subjects can discriminate between two stimuli X and Y. I conducted an experiment on randomly selected 10 subjects. Each subject performed 20 trials. In each trial he/she had to decide if the stimuli is X or Y. The probability of each stimulus was set to 50% (equal number of X and Y so guessing should result in 0.5 success rate). We plotted the result as below: Red circle means a subject that can significantly discriminate X from Y (P < 0.05). Black is mean and blue is median of all subjects success rates.

how to decide if a sample should be colored in red?

how to decide if the mean and the median are significantly different from chance level (e.g. 0.5)?